At the bottom of Earth's orbit

By Phil Plait | January 3, 2011 7:10 am

[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn’t a math error but a typographical one, and I used the right value when doing my calculations below.]

Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month.

But don’t panic! We do this every year. And that part of it ends today anyway.

The Earth’s orbit around the Sun is not a perfect circle. It’s actually an ellipse, so sometimes we’re closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year — you can get the numbers at the Naval Observatory site — but aphelion (when we’re farthest from the Sun) happens in July, and perihelion (when we’re closest) in January.

And we’re at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we’ll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun.

Of course, since today is when we’re closest to the Sun this year, every day for the next six months after we’ll be a bit farther away. That reaches its peak when we’re at aphelion this year on July 4th, when we’ll be 152,096,155 km (94,507,988 miles) from the Sun.

Not that you’d notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:

This may seem a bit odd if you’re not used to the physics of orbital motion, but you can think of the Earth as moving around the Sun with two velocities: one sideways as it sweeps around its orbit, the other (much smaller) toward and away from the Sun over the course of a year. The two add together to give us our elliptical orbit. The sideways (what astronomers call tangential) velocity is about 30 kilometers (18 miles) per second, which is incredibly fast. But then, we do travel an orbit that’s nearly a billion kilometers in circumference every year!

The velocity toward and away from the Sun (what we call the radial velocity because its direction is along the orbital radius) is much smaller; only about 0.3 km/sec (which translates into the numbers I used in the first paragraph above). That’s an average over the course of the year which I estimated very simply by taking the difference between our aphelion and perihelion distances — almost exactly 5 million km (3 million miles) — and dividing by the time it takes the Earth to move between them: half a year, or about 182 days. The exact speed changes, because at perihelion, we’re closer to the Sun and feel its gravity a bit more strongly, so our speed around the Sun is a bit faster than at aphelion.

Together, the tangential and radial velocities add up to gives us our overall orbital velocity, which changes with distance from the Sun. In fact, at perihelion today we’ll be moving around the Sun at 30.1 km/sec, and at aphelion in July that will have slowed to about 29.6 km/sec. That’s a change of about 1.7%; enough to measure if you have the right equipment, but not anything you’d notice in your daily life.

This does bring up another interesting point: when we’re closer to the Sun we receive more light — and therefore energy and heat — from it than when we’re farther away. We can calculate that as well. The amount of energy you receive from an object gets smaller with the square of your distance: double your distance and you only get 1/4 the amount of light from it. Go 10 times farther away and that drops to 1/100 or 1%. At aphelion, we’re 1.033 times farther from the Sun, so we get (1.033)2 or about 1.07 times less light and energy from it. You can flip that around to say that today we are receiving about 7% more sunlight than on aphelion in July!

That may seem weird to folks living north of the equator, but seasons are a whole ‘nuther issue.

Oh, and hey, one more thing. Every now and again I’ll hear from a kid or parent who tells me that they had a teacher or friend claim that if the Earth were even a few thousand miles closer or farther from the Sun we’d burn up or freeze. That’s clearly silly, since over the course of six months the Earth’s distance to the Sun changes by 3 million miles! Not only that, but the Earth is 8000 miles (13,000 km) across and spins once a day. That means at noon you’re 8000 miles closer to the Sun than you are at midnight, and I don’t general see people bursting into flame and then freezing in a block of ice every 12 hours. So if you ever hear that particular bit of silliness, refer ‘em here.

So there you go. You may not notice the Sun looking slightly bigger, or being warmer, or moving faster* than usual today, but it is. So if you’re having a tough day, remember this: it’s all uphill from here.

Until July.



* Relative to the stars it’s moving faster, that is, since if you were to measure its speed across the sky as it rises and sets, the Sun would actually appear to be moving more slowly, because as you stand on the Earth its spin moves the Sun left to right relative to you (if you’re in the northern hemisphere facing south, or standing on your head in Australia facing north) making a single circuit across the sky once per day, while the Earth’s orbital motion moves the Sun right to left relative to the stars making a circuit once per year, with that motion fastest at perihelion, therefore subtracting from or slowing the diurnal (daily motion) of the Sun left to right, so the Sun appears to move in the sky most slowly at perihelion.

Got it?

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

Comments (71)

  1. Dr. Plait, stop spreading those lies. There is no way we oscillate by millions of miles! I saw it on facebook!

    http://www.facebook.com/photo.php?fbid=10150343395495372&set=o.138137079569590

    ;)

  2. Yeah, teachers can say the darndest things. :) Good synopsis Phil!

  3. s. pimpernel

    P.Z. has a post today about some creationist saying a change of 10 feet either way would have us either frying or freezing and a person’s comment debunking that as you did.

  4. Kelly

    Maybe Phil saw the following related item at Pharyngula today?

    http://scienceblogs.com/pharyngula/2011/01/the_science_vs_creationism_deb.php

    (I have no comment about a Biologist commenting on Astronomy!)

  5. zeke

    The religious types don’t like to think critically — which is why they’re religious, I guess.

    http://www.facebook.com/photo.php?fbid=10150343395495372&set=o.138137079569590

    Note the response to the fact-checker….

    Sigh.

    zeke

  6. Brian S.

    …and then the lord said let there be perihelion, and there was, and it was good… and confusing for elementary teachers.

  7. Chris

    Don’t forget we sweep out equal areas in equal time.

  8. Daniel J. Andrews

    Oh yeah…if we’re further from the sun in the summer then why is summer so much warmer than winter, huh? You so called experts are going to be exposed for the frauds that you are. You’re just hiding the truth so you don’t lose your grant money.

    Thinking experts have overlooked some basic bit of physics. Check.*
    Refusing to follow explanatory links. Check. *
    Conspiracy theory. Check.*

    My antiscience certification will arrive any day now.

    *Sadly, we’ve all seen examples where people do these very things and repeat assertions long since debunked by basic science (e.g. physics and chemistry) whenever Phil posts on a hot topic.

  9. tmac57

    @Chris-I dunno,I just swept out my garage,and it took me twice as long as it did to sweep my living room,and they’re about the same size.Damn cobwebs!!!

  10. There is the operational part. The Equivalence Principle (EP) postulates all bodies vacuum free fall along identical minimum action trajectories, that gravitational and inertial masses are fundamentally indistinguishable. If your EP experiment’s output arises from Earth’s inertial acceleration (spin) versus its gravitational acceleration (orbit), 03 January is the best day! (The sweet spot is 44.952 latitude, from WGS84) 03 January is also the hot tamale for the Nordtvedt effect and lunar laser ranging.

    Your solar axion telescope will detect nothing on 03 January 2011 1900Z.

  11. Phil,

    I think that the perihelion distance that you’ve quoted are a bit off. Wikipedia (yeah, I know) lists:

    Perihelion: 91,402,505 miles (147,098,074 km)

    Aphelion: 94,509,130 miles (152,097,701 km)

    This gives the 3.1 million miles difference, whereas the differenc on yours is 94,507,988 miles – 92,955,807 miles = 1.5 million miles.

  12. Dan I.

    @ 8. Daniel J. Andrews

    Be careful you’re treading real close to Poe’s Law there my friend.

    Poe’s Law: Without a winking smiley or other blatant display of humor, it is impossible to create a parody of Fundamentalism that SOMEONE won’t mistake for the real thing.

  13. bigjohn756

    Phil, even your graphic has perihelion listed as 147.1 million km. I think that you have used the average distance in your article.

  14. Bill Nettles

    Yeah,
    Phil quoted the semi-major axis value when he said “Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we’ll be about 149,597,871 kilometers (92,955,807 miles) from the Sun.”

    I’m sure he’ll fix it before he turns his final draft in.

    Regarding the “facebook” photo that’s floating around in the comments, why is there a “Failbook.com” tag at the bottom. Makes me think someone invented the whole scenario just to make a joke. I can’t take that seriously (as PZ shouldn’t have) unless you give me the actual reference (i.e., a real Facebook page), not some redacted secondary source without names and faces.

  15. T. Miller

    7% increase in insolation is incredible! That is so significant that taking a picture (of Earthly things) at perihelion vs. aphelion requires a different exposure.

  16. Shoeshine Boy

    Does anybody know where I can find historic dates for perihelion? I know it varies slightly from year to year, but I just might have been born on a perihelion date. :)

  17. idahogie

    I was confused by this sentence in your paragraph about our radial velocity:

    “The exact speed changes, because at perihelion, we’re closer to the Sun and feel its gravity a bit more strongly, so our speed around the Sun is a bit faster than at aphelion.”

    WRT radial velocity, isn’t our speed zero at perihelion and aphelion? I think that your sentence actually refers to the combined velocities (radial and orbital), although to actually talk about combining them in the following paragraph.

  18. cgauthier

    (I have no comment about a Biologist commenting on Astronomy!)

    PZ wasn’t commenting on astronomy… the person on that Facebook page was. PZ was commenting on the way religiosity erodes critical thought and how frustrating it is to argue with a believer. An comment
    that some, otherwise stalwart, skeptics are squeamish about making (publicly), because of how gosh-darn awfully mean it is to point out the stupidity and harmful influence of the single most widely held delusion on earth.

  19. Jason

    @16 Shoeshine Boy. that would be interesting. I am looking for a site that can calculate or historically show perihelion. I haven’t found it yet.

  20. Chris

    @Zeke – Is that true? I can’t believe people would be that stupid. Well 500 million people on facebook, There would be ~675000 people with an IQ below 70, so it is possible. At least that person has a smart friend.

  21. Pete Jackson

    A cop pulled me over today for speeding, and I explained that this is the day when the earth travels the fastest in its orbit, so that you have to move faster as well to keep from being left behind!

  22. Matt

    Back in school, I was called a liar for saying the Earth was closest to the sun in Winter. Too bad I can’t remember who exactly it was who said that, so I could send him this link.

  23. DrFlimmer

    That’s just another reason why tomorrows solar eclipse is only partial with the sun being very close by and the moon very far away. Relatively speaking.

    I hope those damn clouds dissolve during the next 12 hours….

  24. MadScientist

    In other words: why coronagraphs have a minimum of 3 occulting cones.

    @Matt#21: However, that is not true in the opposite hemisphere, nor will it be true in a few thousand years as the earth’s axis precesses.

  25. mike burkhart

    In one episode of The Twilight Zone , the Earth (for resions that are not explained) has changed its orbit and is headed toward the sun a woman wacthes as it gets hoter and then passes out and wakes up in bed to learn she had a fever , but its reveled that the Earth (again no resion ) is really headed away from the sun. This was simular to a 1960s movie called the Day the Earth Caught Fire in witch after the most powerfull nucler wepon is tested the explosion koncks the Earth in to a orbit that takes it closer to the sun. I would say this ,the Earth has been in this orbit since it formed 4.5 billon years ago and it will most likely be in it untill its end. Off Topic : I saw a fireball last night wile observing .

  26. As Phil states, Earth moves slightly faster in its orbit around the Sun at this time of year. That also slightly lengthens the solar day since Earth has to rotate a little bit extra to reach the next local noon. 24 hours is the mean solar day, but sometimes solar days are a little bit longer and sometimes a little bit shorter.

  27. Nemo

    @Bill Nettles #14:

    I haven’t seen the original of the “10 ft” one, but in the comments to that post on Pharyngula, there’s a link to a Facebook page with an even worse figure: 8 inches. (It’s buried in a reply to a long global warming denialism screed, but it’s there, and appears sincere.)

  28. In honor of the occasion, I started a #PerihelionMyths hashtag on Twitter. It’s almost guaranteed not to trend.

  29. Bill Nettles

    idahogie #18:
    I agree that Phil mixed his ideas here. The radial velocities at the aphelion and perihelion are zero. He does use the word “speed,” however, but again didn’t clarify that he was talking about total when the context was radial.

    I have a bigger problem, one that we physics teachers try to convince our students about, and that Phil just set us back about 10 perihelions. “The exact speed changes, because at perihelion, we’re closer to the Sun and feel its gravity a bit more strongly, so our speed around the Sun is a bit faster than at aphelion.” He mixes 3 facts, ignores a definition and states a conclusion that is factual, but NOT logically related.
    Fact 1: exact speed changes
    Fact 2: closer to the Sun
    Fact 3: feel (“Feelings, nothing more than feelings … ahem) its gravity a bit more strongly
    Ignored definition: acceleration (directly related to the gravity) tells us the CHANGE in velocity, not the velocity. Bigger acceleration magnitude DO NOT mean bigger speed! Phil, know this! Plus, big accelerations can actually make for smaller speeds, like stomping on the brakes in your car, or an alpha particle approaching a gold nucleus.
    Factual conclusion: our speed around the Sun …
    NOT logically related: the “so” is wrong! see the Ignored definition for the reason.

    Better explanation: the angular momentum of the Earth-Moon system around the Sun is constant (to a very good approximation, ignoring other planets). The angular momentum depends on the distance and the velocity; at smaller distances that velocity has to get bigger so that the angular momentum is conserved.

  30. Bill Nettles

    Nemo #28:
    Thanks. Don’t have a way to judge the sincerity, but people writing in screeds tend to just “mouth off” about things.

    The fact still remains that there is no source given for the “page” that PZ is writing about. I’m interested in seeing the original source material, not a redacted photo from “failbook.com” Maybe I should check wikileaks, but then, according to conspiracy nuts, I’d be banned from government employment forever.

  31. Because the day is longer now, events are retarded – for instance, sunrise is slightly later, as is sunset. Note that the shortest day is Dec 21 so sunset is getting later (slightly) ANYHOW, but people who say that Dec 21 is the latest sunrise are wrong. Latest sunrise is about New Years day.

    I will be most happy to see earlier sunrises. Short days depress me. Since I usually get up about 6AM, earlier sunrises will be welcome.

  32. Bill Nettles

    Matt #23:
    Just make sure you weren’t living south of the Equator when that confrontation happened.

  33. cy

    OMG! Earth now falling away from Sun at 1100 km/hr!

    Its must be the the gravity of Niburu!

    Good article as always Phil.

  34. Ross

    Off topic, but a great example of ordinary people doing extraordinary science:

    http://www.thestar.co.uk/diary/New-planets-are-a-gas.6672280.jp

    Amateur astronomer discovers 4 new planets by analyzing publicly released astronomical data.

  35. Chief

    I found myself unable to walk a straight line today, Thanks to Phil I now know it’s because we are closer to the sun and I was being pulled off course.

    Been watching NOVA (on climatic change and global CO2) and the current thread has gotten me thinking about the axis tilt. It varies though the millions of year cycle and I wonder what the timing effect on past climatic changes. Less inclination, more heat year around.

  36. Anchor

    “Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month.”

    Oy.

    No, the figure of approach is actually closer to the AVERAGE over that time. We were easing slowly away from aphelion at first, reached a peak speed of approach to the Sun in October near the semi-major distance, and have been slowing up to reach perihelion. Now we’re slowly easing our way back out towards aphelion again, but its just not accurate to put a single number to the value of approach and recession during the course of an elliptical orbit. That’s like saying a pendulum has only one speed between its widest excursions during its swing. You know better than that.

  37. Joseph G

    Dumb question: Is this why max and minimum temps seem to be more extreme in the southern hemisphere? Since their solar “winter” and “summer” coincides with aphelion and perihelion, respectively?

  38. Meskine

    As a resident of Texas, how happy am I that aphelion occurs in July? I’ll have to remember to bundle up when I go out to mow the yard next summer.

  39. AliCali

    @ Anchor (37):

    “No, the figure of approach is actually closer to the AVERAGE over that time…That’s like saying a pendulum has only one speed between its widest excursions during its swing. You know better than that.”

    I think Dr. Plait does know better, since he wrote, “The velocity toward and away from the Sun…is…about 0.3 km/sec….That’s an AVERAGE over the course of the year which I estimated very simply by taking the difference between our aphelion and perihelion distances — almost exactly 5 million km (3 million miles) — and dividing by the time it takes the Earth to move between them: half a year, or about 182 days…” (I emphasized ‘average’).

    Also, he’s showing general concepts, not exact math. If Dr. Plait includes all the little variables, his post will be way too long for the point he wants to make.

  40. Monkey

    You state the difference is ~149 million km to ~152 million km; the 2005 images states ~147 – ~152 million km. Is there a difference each year (obviously it wont be exact to the decimal place) that we need to measure each year, as in “this year aphelion will be X million km compared to last years Y million km”? A difference of 2 million km must make some difference?

    Good article, as always..

  41. Monkey

    I used to always get them mixed up – then I made a little game of it. Longer word = shorter distance. Opposites, and I remember forever!

  42. Chew

    I love these astronomy lessons. Reminds me of Phil’s original pantry. More, please.

  43. Craig

    Maybe a dumb question…..If we’re that much closer to the Sun, is the daylight measurably brighter?

  44. Messier Tidy Upper

    @^ Craig : Measurable with *what*? ;-)

    Great photo comparison – thanks BA & Anthony Ayiomamitis. 8)

    Would I be right though in thinking I’ve seen something like this before? Maybe last year or a few years ago? Or even last aphelion?

    Not that I mind if so!

    @22. Pete Jackson Says:

    A cop pulled me over today for speeding, and I explained that this is the day when the earth travels the fastest in its orbit, so that you have to move faster as well to keep from being left behind!

    LOL. :-D

    @33. Bill Nettles :

    Matt #23 : Just make sure you weren’t living south of the Equator when that confrontation happened.

    Why? Perihelion is the same for *both* hemispheres.
    [Imagines two half Earth’s Northern & Southern travelling around our Sun at opposite points in the one orbit ie. one at perihelion, t’other aphelion, nah! Wouldn’t work.]

    @38. Joseph G Says:

    Dumb question: Is this why max and minimum temps seem to be more extreme in the southern hemisphere? Since their solar “winter” and “summer” coincides with aphelion and perihelion, respectively?

    My understanding is that this has more to do with the distribution of continents / oceans and land-sea ratio ie. much more land in the northern hemisphere than the Southern but I’m not entirely sure & could well be wrong about this.

    Also, fixed that for you, not a dumb question at all. ;-)

  45. Messier Tidy Upper

    @14. Bill Nettles :

    … Regarding the “facebook” photo that’s floating around in the comments, why is there a “Failbook.com” tag at the bottom. Makes me think someone invented the whole scenario just to make a joke. I can’t take that seriously (as PZ shouldn’t have) unless you give me the actual reference (i.e., a real Facebook page), not some redacted secondary source without names and faces.

    It *is* sometimes hard to believe just how stupid some people can be but evidence comes via this comment (#50) from ‘hje’ on pharyngula :

    http://scienceblogs.com/pharyngula/2011/01/the_science_vs_creationism_deb.php#comment-3067602

    & from there this :

    http://www.origins.org/articles/adamson_isthereagod.html

    Would appear to be the likely primary source perhaps via being quoted at a sermon or from someone else mentioning it -& of course not checking with anyone who might actually know. :roll:

    Also this link via Ichthyic’s comment (#67) on the relevant pharyngula thread :

    http://funnyjunk.com/showcomment/14141214/

    Provides a classic comic rebuttal of the silly notion too! Make sure you scroll down. Although, wouldn’t you need a longer ladder? ;-)

  46. I’m loving this writeup. Never seen photos comparing apparent sun size at the apsides before…

    I took a lighter approach in writing about the perihelion. Launched my Sun appreciation art campaign today.

    Folks may be interested, I’d love to hear feedback: http://SUNWORSHIP.info/2011/perihelion/

  47. Messier Tidy Upper

    @26. mike burkhart :

    .. I would say this ,the Earth has been in this orbit since it formed 4.5 billon years ago and it will most likely be in it until its end.

    [Pedant mode on] Actually, that’s not quite accurate. As our Sun loses mass especially towards the end of its main-sequence life and into its red giant phase we have good reason to think that Earth’s orbit will probably shift outwards.

    There’s also the slim (~1% or so if memory serves) chance that our inner solar system planets incl. Earth could have their orbits destabilised and perhaps even end up colliding or being ejected because of Mercury & Venus chaotically interacting gravitationally. :-o

    That’s assuming we don’t move it further out of harm’s way ourselves as suggested in a space-dot-com article. ;-)

    Off Topic : I saw a fireball last night while observing.

    Excellent! Nothing quite like a decent fireball in the night. Love it when I look up at just the right time to observe that. :-)

    @33. Bill Nettles : D’oh! Having just checked (too late natch) I’ve realised that while I’d mistakenly thought you were responding to the confrontation with the police officer by (#22.) Pete Jackson, you were *really* responding to a confrontation over where our Sun / Earth is in season by (#23.) Matt instead. Mea culpa. My apologies.

    [Puts it down to my brain still being affected by the traditional New Year’s Day hangover & still not working right yet. ;-) ]

  48. Messier Tidy Upper

    See : http://kencroswell.com/MercuryCrash.html

    For the potential distant future problems with Mercury – note from there :

    “…the good news: there’s only about a 1 percent chance that Mercury will go crazy before the Sun bloats into a red giant billions of years from now. “If you’re an optimist,” says Laughlin, “then you say the glass is 99 percent full.”

    This link gives one source :

    http://www.space.com/scienceastronomy/solarsystem/death_of_earth_000224.html

    on Earth moving out as the Sun’s stature shrinks. Although it mentions that :

    “If the sun loses mass before it gets too big, then Earth moves into a larger orbit and escapes,” Willson told SPACE.com. “The sun would need to lose 20 percent of its mass earlier in its evolution, and this is not what we expect to happen.”

    Also, elsewhere it has been suggested that the Earth may raise a tidal bulge in the red giant Sun’s surface causing our world to eventually spiral inwards to destruction & there still seems to be some uncertainty over our globe’s eventual fate.

    Also here :

    http://www.space.com/scienceastronomy/planetearth/earth_move_010207.html

    is the space.com article mentioned about us moving Earth further from the swelling Sun. Talk about ambitious plans – & forward thinking, long term ones at that! ;-)

    Then again, Anaxagoras (or was it Pythagoras? Or another classical Hellenic thinker?) once famously (sorta) suggested it first :

    “Give me a lever long enough and I’ll move the Earth!”* ;-)

    * In Ancient Greek naturally & also may not be exact quote. (Typing from memory only.)

  49. Anchor

    AliCali #40 says, “I think Dr. Plait does know better, since he wrote, “The velocity toward and away from the Sun…is…about 0.3 km/sec….That’s an AVERAGE over the course of the year which I estimated very simply by taking the difference between our aphelion and perihelion distances — almost exactly 5 million km (3 million miles) — and dividing by the time it takes the Earth to move between them: half a year, or about 182 days…” (I emphasized ‘average’). Also, he’s showing general concepts, not exact math. If Dr. Plait includes all the little variables, his post will be way too long for the point he wants to make.”

    Of course he knows better. You might pardon me for re-emphasizing it…because the Good Doctor ;) takes another 7 paragraphs to justify the statement in his opening paragraph, “every moment since then [July], our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month”.

    “Every moment since then…” That is just not true. Unfortunately, by the time he does get around to it, he conflates the rate at which we approached the Sun since last July with our much swifter tangential (orbital) velocity AROUND the Sun at perihelion, which is not directly relevant to the issue. The rest of that paragraph you cite continues with this final sentence: “The exact speed changes, because at perihelion, we’re closer to the Sun and feel its gravity a bit more strongly, so our speed AROUND the Sun is a bit faster than at aphelion.” [Emphasis mine]. That doesn’t address the RADIAL speed component of the Earth’s orbital motion (strictly toward and away from the Sun) over the course of the year.

    Nobody needs to provide any “exact math” featuring the calculus of orbital mechanics (with “all the little variables” as you put it) to succeed in accurately characterizing a general concept. But Phil introduced a point up front then dropped the ball when he had an opportunity to fix it: that the AVERAGE (which he “estimated very simply”) is constant (“Every moment since then…”) is wrong gets a little daylight (“The exact speed changes…”) but then he muddies the radial velocity issue with orbital velocity (“…because at perihelion, we’re closer to the Sun and feel its gravity a bit more strongly, so our speed around the Sun is a bit faster than at aphelion”) and goes on to say:

    “Together, the tangential and radial velocities add up to gives us our overall orbital velocity, which changes with distance from the Sun. In fact, at perihelion today we’ll be moving around the Sun at 30.1 km/sec, and at aphelion in July that will have slowed to about 29.6 km/sec. That’s a change of about 1.7%…”

    – without ever explaining how the Earth manages to snap instantly in RADIAL velocity by over 2200 km/hour every July and January…as explicitly implied by his opening paragraph.

    There isn’t just one point in his post, which is what I like about many of Phil’s posts, no matter how long they are (and many are in fact longer). Instead, he goes on to another point he wants to make, leaving the previous one writhing with a wound in need of proper treatment. It wouldn’t have been at all hard to dispense with the opening phrase in that second sentence of this post: “Every moment since then…” and adding to its tail end, “…on average”. That would have REDUCED his post by 2 words AND satisfied accuracy, which I am sure Phil loves more than dramatic turns of phrase. (Many pardons for raising the issue, Phil).

  50. Mike

    Poem in honor of the occasion:

    Mother Sun holds the baby Earth
    Close with her loving potion.
    To explain it all, for what it’s worth –
    It’s nought but relative emotion.

    :)

  51. Mike

    Is it just me, but I don’t think 152,096,155 minus 149,597,871 equals five million? 800,000km per month might make sense if Earth travelled from perihelion to apohelion and back in half a year, but I think it only goes from the closest to farthest away in six months and if the distances are correct (149Mkm and 152Mkm) then the speeds must be off. Or vice versa.

  52. Michel

    This mornig (jan 4) we could enjoy a partial eclipse.
    First photo is me with our village and the “Monte Toro” in the background. Right of the antannas there are still some clouds, but the sun isn´t up yet. Every minute the clouds moved on north (to the left) and the the sun came up. And it was a double upper. The moon was already in frot of the sun so we first saw two point of light coming rising over the ridge. So it was abeautiful sunrise.
    Photo two is my very first ecplise photo ever.
    1:
    http://cid-2a6ed024f849b373.photos.live.com/self.aspx/NexStar%20SLT%20130/P1041383.JPG
    2:
    http://cid-2a6ed024f849b373.photos.live.com/self.aspx/NexStar%20SLT%20130/Eclips%2004012011%200854.jpg

    What a lovely way to start the first working day of the new year!

  53. chris j.

    this post is a great starting point for two major sun/solar orbit related topics.

    first, the “few thousand miles between burning and freezing” part is clearly the result of reading too much into the “goldilocks zone” where the solar radiation the earth receives is “just right.” people can easily jump to the conclusion that the earth is in exactly the right place, and that there is no margin for error, when in fact the zone is many millions of km wide, and depending on whom you ask, may include venus and mars as well.

    second, many global warming denialists claim that the recent warming trend can be attributed to increased solar output, writing off anthropogenic sources of CO2 and other GHGs. but they can’t account for this periodic shift in the amount of sunlight we receive, meaning that the composition of our atmosphere is vastly more significant than the relative quantity of light we get from the sun, and any variations in that quantity.

  54. Messier Tidy Upper

    @51. Mike : Poem or song lyrics? ;-)

    Seems vaguely familiar from somewhere – maybe?

    @53. chris j. :

    people can easily jump to the conclusion that the earth is in exactly the right place, and that there is no margin for error, when in fact the zone is many millions of km wide, and depending on whom you ask, may include Venus and Mars as well.

    Don’t forget Europa either! ;-)

    Possibly Enceladus and Mars still – the main problem with Mars being its low mass rather than its orbital position or so I gather.

    It has been speculated that Venus may have life in certain cloud layers (read Ben Bova’s eponymous novel) and the same also goes for the gas giants with Clarke postulating living creatures floating inside Jupiter as well as Europa in the Space Odyssey novels. Scientists (& SF writers) have speculated about life on Titan and Enceladus and inside comets and even in deep space itself so ..? Well, there’s a lot of ideas and guesswork and little clear evidence in my understanding of things here.

    I think our current level of knowledge is insufficent to really determine whether the notion of a Habitable Zone (HZ) is all that valid or not. Life may or may not be like us & like we imagine it might be.

  55. Shoeshine Boy

    For those looking for perihilion dates in history, I located the following website with help from some friends at the Official Bad Astronomy and Universe Today Forum (bautforum.com). Just replace “1844” with the desired date in the yellow box at:

    http://m.wolframalpha.com/input/?i=earth%20Perihelion%201844

  56. Bobby Stefanov

    Thankfully where I live the clouds dispersed just enough for us to enjoy the 3% larger Sun eclipsed by the Moon.
    One of my photos turned out good, so I thought I’d share – http://www.deviantart.com/download/192141948/partial_eclipse_of_jan_2011_by_lordnosferatu-d36e9oc.jpg

  57. Bill Nettles

    46. Messier Tidy Upper:

    The primary source than I’m looking for is the unredacted Facebook page, or the name of the real people involved in the situation, not some 3rd or 4th level screen shot. That seems like something that is made up for a joke. There is nothing real about it except the appearance, and appearances can be deceiving even when they appear to support a particular viewpoint. PZ wants theists to appear stupid, so he picks up on a screenshot that he likes and treats it like fact. His article is just as flawed factually as the straw man screenshot that someone has set up. Did he check to see whether the screenshot was real? He never says that he did, and he doesn’t reference any real primary source. But all his congregation said, “Amen!” Bad science.

    Still waiting for someone to reproduce the original Facebook source (page location or person).

    Glad you recognized I wasn’t commenting on policemen but on seasons with Matt.

  58. Joseph G
  59. Joseph G

    I was fortunate to see PZ Meyer personally recently when he gave a talk at UCSC (I don’t go there but it’s my home town and I grew up here, so dammit, I went). It was quite entertaining, but I have to say, there was a palpable air of smugness in the room. The talk was less about exactly why it’s important that people are educated about evolutionary theory and more about how stupid YECs are (to be fair, they can be quite ridiculous). I certainly agree with his overall message, but I found some of the audience to be just plain insufferable.
    Of course, since I was part of the audience, what does that say about me? :D

  60. Matt B.

    I think the claim that being a few thousand miles closer to or further from the Sun would burn or freeze the Earth is about the size of the orbit being different, not about how far the Earth is at a given moment. That still doesn’t mean it’s correct, but we can give the benefit of the doubt.

  61. mike burkhart

    I like the Twilight Zone ,may of its storys have social commentary in them (like Star Trek) and Rod Serling beeing a World War 2 vet hardly supriseing that a lot of episodes were about WW2.But one problem with the show is scientific accuracy .Big example having planets only 1millon miles form Earth ,Venus isn’t that close there are many others

  62. Shoeshine Boy

    @#57 Bobby Stefanov: Nice photo!

  63. Mike

    @55 Messier – Not lyrics, it’s my poem.
    @59 Joseph – Thanks!

  64. darthpickley

    Sucks to be Australia right now, having perihelion combined with their winter solstice (Jun), and aphelion combined with their summer solstice (Dec). Must be extreme there.

    … or is it? An interesting effect comes into play when one considers:
    Asymmetries of water/land in north (60% water) vs. south (80% water) hemispheres
    Water absorbs and releases heat more slowly than land.
    Total effect on global Climate

    ok, now given these effects… I have no clue what the effects will be.

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