Does the Sun look smaller to you?

By Phil Plait | July 4, 2008 1:30 pm

Criminy, I almost forgot: today, July 4th, at roughly 08:00 UT, the Earth was at aphelion.

Uh, what? I hear you ask. OK, brief astrolesson for ya, then back to the grill!

The Earth does not orbit the Sun in a perfect circle. The orbit is slightly elliptical. If you were to draw the Earth’s orbit on a piece of paper, you’d need a sharp eye to detect its non-circularity, but deviant it is. What this means in real terms is that the Earth ranges from about 148 to about 152 million kilometers from the Sun over the course of six months (which is how long it takes to get from one side of the orbit to the other, of course).

When the Earth is closest to the Sun it’s at perihelion, and when it’s farthest it’s called aphelion (I usually pronounce that app-helion, if you care, though I’ve heard others say aff-helion). So today we passed aphelion, and slowly but inexorably, over the next six months we’ll draw slightly closer to the Sun, and then the whole thing repeats.

That 4 million km difference sounds like a lot. But over the 150 million average radius of the orbit it’s only a slight difference by eye. The Sun will look about 3% larger at perihelion versus aphelion, and you’d never notice that, especially since the change is slow and takes six months. The amount of sunlight hitting the Earth does increase at perihelion, being about 5% greater than at aphelion. That’s quite a bit! But the effect isn’t as bad as you’d think. Why not?

For us northern hemisphere folks, we are farthest from the Sun in summer, and closest in winter, so that mitigates the temperature extreme. On average, winters are a bit warmer and summers a bit cooler. But wait! In the southern hemisphere, the seasons are reversed! So they should have extra hot summers and extra cold winters.

But they don’t. Why not? Because the southern hemisphere is mostly water. Go ahead, find a globe and take a look; it’s incredible how much of that half the Earth is water bound. Water absorbs and releases heat slowly, so all summer the oceans suck down that extra solar energy, and release it all winter. That helps balance out the temperature extremes.

Oh, one more thing: the Earth precesses, that is, the axis of rotation moves like a wobbling top. It takes a long time for the wobble to make one cycle, well over 20,000 years. But this changes the timing of the seasons compared to the orbit. In a few millennia, we’ll have perihelion at the same time as northern summer, and aphelion at northern winter. It’s hard to say what effect this will have on the environment, since it brings extra-hot summers and extra-cold winters. However, the last time this happened was around the same time the Sahara forest went away and was replaced by, well, guess.

But for today, don’t fret too much about wandering poles and aphelion… except to say, if you’re out sweltering in the Sun today celebrating the holiday in the U.S., you might want to take just a moment and be glad our orbit isn’t more elliptical, or that it isn’t 15,000 AD.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (45)

  1. Of course the precession of the Earth’s axis will also change the position of the stars in the sky relative to geographic north.

    It’s a good thing we’ve got GPS ’cause the north star won’t be much use for navigation in fifteen thousand years or so.

    Seriously, I didn’t know that the climate was meaningfully affected by planet’s orbit, nor that this effect was mitigated by continental distribution. I am thusly smarter and in your debt.

  2. ioresult

    Back to the BBQ, the hotdogs are burning!

    PS: this new blog format doesn’t display the time at which you posted, just the date, so I can’t say if it’s plausible to tell you to go back to your burning sausages.

  3. Blaidd Drwg

    Of course precession also means that astrology is fundamentally wrong, since the signs that were at zenith when astrology was originally defined are now shifted, ergo, if you think you are a Scorpio because your b’day is in November, you are actually a Virgo. December babies are the TRUE Scorpios, etc.

    Not to mention the varying distance to the Sun has an effect on lunar eclipses, along with the variable distance to the moon, negating at least one of the creos’ favorite claims, that the distance has to be PERFECT or we would never see a total solar eclipse (In fact we see lots of solar eclipses, some are annular, some have ‘perfect’ coverage, some have the moon actually appear bigger than the sun).

    OT: One day to Doctor Who season finale’! (It’s bittersweet, since there will be no more for almost 2 years, other than the token specials)

    (Can somebody give the Doctor a hand? I’m just sayin’…..)

  4. Tim G

    Mars is at aphelion about the same time as its northern summer as is with earth. Its orbital eccentricity is more significant and receives 45% more sunlight at its perihelion. Its southern hemisphere seasons are more extreme as expected and of course there’s no major bodies of water to moderate climate locally.

    I think it would be neat to live on a habitable planet with a much more pronounced orbital eccentricity with aphelion (apastron?) coinciding with a solstice. You could have a wider variety of climates represented.

    (It should be pronounced app-helion because there’s a helios in there, right?)

  5. Andy Beaton

    If it were 15000 AD, we’d be living in colonies on the Jovian moons, where the orbit of the Earth would not concern us. Or we’d be extinct for 12988 years.

  6. Sili

    Yes it should, Tim G.

    I think some people are tricked by the ‘ph’ digraph. I find it hard to believe they actually pronounce the ‘h’ too, though. I wonder if they don’t just say /ə’fiːlɪən/.

  7. When the Earth is closest to the Sun it’s at perihelion, and when it’s farthest it’s called aphelion

    I keep it sorted out by thinking that at perihelion the two objects are more of a pair.

  8. Precession has moved the positions of the stars over half a degree within my lifetime (or Phil’s since we’re about the same age). I always find that a bit boggling when I think about it… the whole sky shifting. Everything has moved more than the diameter of the full moon.

    And that means the seasons have changed, ever so slightly, in that time :)

  9. llewelly

    Seriously, I didn’t know that the climate was meaningfully affected by planet’s orbit, nor that this effect was mitigated by continental distribution. I am thusly smarter and in your debt.

    If you want to know more about how variations in orbit affect climate, you should read up on Milankovitch cycles. Try wikipedia for a cheap start. More in-depth info can be found on RealClimate, for example here . Much easier to read than any web article is the slightly dated but very good book, Ice Ages: Solving the Mystery by Imbrie & Imbrie.

  10. quasidog

    Wow that is amazing. Again the earth is in a ‘right place, right state, right time’ situation. I never knew that about the temperature balancing out like that. I knew the orbit was slightly eliptical but it never occurred to me what effect that would have on temperature, and how even though there are more variables here than I realised, it is still in balance all over. I thought seasons were cool, but this is cooler. Great post BA.

  11. Craig

    Wow that is amazing. Again the earth is in a ‘right place, right state, right time’ situation. I never knew that about the temperature balancing out like that.

    Yes, I know what you mean. It’s too good to be true, isn’t it? Almost like some intelligence had designed it that way… <evil grin>

  12. Ronn! Blankenship

    It’s raining here . . .

  13. ErrolC

    It’s a good thing we’ve got GPS ’cause the north star won’t be much use for navigation in fifteen thousand years or so.

    Well the Polynesians managed fairly well a couple of millennia ago, I’m sure we’ll cope. How far south can you see the “North Star” anyway?
    BTW, pre-teens are taught how to find celestial south down here (New Zealand), it’s not too hard.

  14. Ronn! Blankenship

    Seriously . . . I’ve found that when I tell an introductory class that Earth is closest to the Sun in early January and farthest around the 4th of July pretty much every semester at least one person will raise their hand and say, “Don’t you mean the other way around?”

    (Guess I wouldn’t get that if I taught in the Southern Hemisphere . . . )

  15. Ronn! Blankenship

    Blaidd Drwg Says:
    July 4th, 2008 at 1:58 pm

    “Of course precession also means that astrology is fundamentally wrong, since the signs that were at zenith when astrology was originally defined are now shifted, ergo, if you think you are a Scorpio because your b’day is in November, you are actually a Virgo. December babies are the TRUE Scorpios, etc.”

    Except for the 2/3 of them born when the Sun is in Ophiuchus . . .

  16. CL

    Happy 4th from Flagstaff, Arizona! Thanks for the astrolesson…nothing better than a little science education on the holiday.

    http://www.coulterlewkowitz.com

  17. Steve

    I always wondered why winters in southern Australia weren’t colder – thanks, Phil.

    ErrolC, how do you southerners find celestial south?

  18. Francisco Burnay

    Greetings from Portugal!

    It should be mentioned that when the Earth is closer to the Sun, its orbital velocity increases. This means that the extra energy our planet gets from the Sun is compensated by the smaller period of time it stands closer to the radiation source. Actually, they even out exactly, for both phenomena obey to a inverse square law. Although pretty obvious, this question was in the center of the problem of explaining the cycles of temperature of our planet – Adhémar’s cataclismic model was proven incorrect by Humboldt’s mention of d’Alembert’s work. This story is explained in John Gribbin’s excellent book “Science: a History 1543-2001″.

  19. llewelly

    Of course precession also means that astrology is fundamentally wrong, since the signs that were at zenith when astrology was originally defined are now shifted, ergo, if you think you are a Scorpio because your b’day is in November, you are actually a Virgo. December babies are the TRUE Scorpios, etc.

    Precession often tends to be a good bullet in the head for many ‘ancient astronaut’ or ‘these pyramids are aligned with Orion’s dingus’ type notions. If you ever find the claim that old buildings or old inscriptions represent a certain set of stars, trawl the web for an astronomical analysis of where those stars were in the sky (relative to Earth, of course) when the buildings were built or when the inscriptions were made. If the claim never references any such analysis, or the analysis you find indicates the claimants did not take into account the Earth’s myriad changes in axis orientation, those claims are probably mistaken.

  20. Bill Nettles

    So what are we going to call Polaris when it isn’t the Pole Star any more? “The Star Formerly Known as Polaris”? That’s really a “prince” of a name. (groan) I’m just so glad that 12000 years from now we’ll finally have a really bright pole star – Vega!

  21. Radwaste

    Umm, guys, there’s two kinds of precession. One is where the Earth’s pole doesn’t point at the same place in the sky (Polaris takes the place of Thuban as the North Star), and the other one is the major axis of our orbital ellipse. Eratosthenes found the first one, but not the second.

    Right, Dr. Phil?

  22. Irishman

    Aphelion comes from the Greek apo for “away from” and helios for “Sun”. Ergo, it seems it should be pronounced “ap-helion”. But one website says this:

    (Greek από, from, which becomes απ before a vowel, and αφ before rough breathing)

    http://encyclopedia.thefreedictionary.com/aphelion

    apo, ap, af

    Apparently the h leading into helion is “rough breathing”. This seems strange to me, and it seems more straightforward to say it “ap-helion” in line with “apogee” (farthest distance from Earth) and “apoapsis” (farthest distance from central body being orbited).

    I also note that several definition sites have an audio file to pronounce the word, and all I have found use “affelion”, but oddly it seems they are all linking the same audio file, so not sure what that proves.

    Sili, you are correct, but most of us don’t read formal pronunciation symbols (we need a key), so Phil is trying to represent the pronunciation using English letters. Thus, he is approximating what he hears. The distinction of picking out a hard “h” sound right after an “f” sound is very subtle.

    Bill Nettles said:
    > So what are we going to call Polaris when it isn’t the Pole Star any more?

    I suspect we’ll still call it “Polaris” even though we know it isn’t really aligned with the pole, and just have a historical footnote that at one point it did line up and probably will again.

  23. madge

    The Sun looks smaller and now Mercury has shrunk too!

    http://news.bbc.co.uk/1/hi/sci/tech/7489557.stm

    Aaaaaah! madge runs around in ever decreasing cicles screaming :)

  24. You can see this effect quite nicely with the free planetary software, Stellarium:

    Select the Sun and press space to track it. Then turn off atmosphere, ground, fog and cardinal points (keys a, g, f, q), and switch to equatorial mouse (return key). zoom right into the sun with the page up key until it fills the view. You can then hold the ‘]’ key to quickly advance by a week at a time and you will see how the size of the Sun changes.

    An alternative to the ‘]’ key is to press ‘L’ several times until the simulation speed is fast enough to see the change in size.

    Matthew

  25. I can’t tell if the sun looks smaller or not. In fact, after looking at the sun, I can’t see anything!

  26. madge

    @Miss Cellania
    I was just gonna file a law suit against BA for asking us to look directly at the sun WHICH IS DEFINITE NO NO! Shame on you Phil! From all your now blinded BAbloggees :(

  27. Mateys, you do of course realize that one of you scurvy dogs ought to have addressed why this planet’s wobbly precessional cycle isn’t the reason for that thing that rhymes with “wobal glorming”… ’cause it’s gonna come up.

  28. Radwaste

    Gee, Captain – if the lot of you can’t get the types of precession straight, how can you say one way or the other what the effects are? At least several Ice Ages have cycled through, and the next one is late – during which, the current human population won’t be sustainable.

  29. There has never been a recorded case of total, permanent blindness caused by looking at the Sun with just your eyes. Ever. Damage has either been highly localized and permanent, or general and temporary. I wrote about this extensively in my first book, and it’s mentioned in my second.

    But in the end, really, looking at the Sun makes you squint, makes your eyes water, and hurts. So maybe it’s a good idea not to do it.

  30. aaron

    actually i think it will atke three months not six to reach the closest point. “So today we passed aphelion, and slowly but inexorably, over the next six months we’ll draw slightly closer to the Sun, and then the whole thing repeats.”

  31. Steve Cooperman

    Half a precession cycle from now, we WON’T have summer in December. We follow the “tropical year”, which keeps our calendar in step with the seasons. The GOOD news is that instead of seeing Orion at night in the dead of winter and freezing, we’ll see it in the summer (if you discount global warming when ALL seasons will be hotter!)

  32. Blaidd Drwg

    I KNEW that hand would be important! (Doctor Who). Spectacular way to end the season! But bittersweet, seeing as there will now be a hiatus of almost 2 years before we get our weekly ‘Who fix’ again.

    Craig, this forum is analogous to Dr. Plait’s living room. Keep in mind it is HIS blog, he sets the ground rules. Those who follow the rules are allowed to disagree, even strenuously, provided they remain within the language rules he has set. If you wish to swear, go to PZ’s site, or any of the other blogs that are not ‘kid friendly’.

  33. Tom Marking

    “Oh, one more thing: the Earth precesses, that is, the axis of rotation moves like a wobbling top. It takes a long time for the wobble to make one cycle, well over 20,000 years. But this changes the timing of the seasons compared to the orbit. In a few millennia, we’ll have perihelion at the same time as northern summer, and aphelion at northern winter.”

    The precessional period (25,765 years) of the earth’s axis is not the only effect. It turns out that the perihelion point of the earth’s orbit also precesses with a period of 112,000 years due to gravitational effects of the other planets in the solar system. Relative to the vernal equinox the date of perihelion cycles with a period of 21,000 years. Thus, 12500 CE would have the perihelion point aligned with the summer solstice in the northern hemisphere, not the 15000 CE mentioned.

    http://en.wikipedia.org/wiki/Precession_%28astronomy%29#Anomalistic_precession

  34. Irishman

    Blaidd Drwg, DUDE, don’t throw DR WHO spoilers in non-DR WHO threads. Jeeze, some of us haven’t seen it yet, on account of it not airing here for another couple weeks.

  35. The statement, “Of course precession also means that astrology is fundamentally wrong” repeated by several bloggers is also “fundamentally wrong,” for it applies only to Western (tropical) astrology, which is based upon the spring equinox.

    Vedic Astrology (from India) uses the sidereal, “star based” zodiac which does maintain the connection with the actual astronomical positions. And, this is not some minor branch of astrology but is found both in India and increasingly in the West.

    Science is only well served when all facts are considered.

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