Top o' the orbit to ya!

By Phil Plait | July 4, 2011 5:00 am

Today is the Fourth of July, Independence Day for us American types.

It also happens to be aphelion*, the point in Earth’s ever-so-slightly elliptical orbit when it’s farthest from the Sun. Perihelion — closest approach — happens in early January, and aphelion six months later. The dates change a bit from year to year because there aren’t an even number of days in a year (that pesky extra 0.24 in the 365.24 days per year messes things up), and there are other minor factors as well.

Today though, aphelion occurs on or about 15:00 UT (11:00 Eastern US time), when the center of the Earth will be about 152,102,715 km (94,512,245 miles) from the center of the Sun — give or take a few hundred meters. If you’re curious, that’s about 1.67% farther from the Sun than on average. That in turn means the Sun appears about 1.67% smaller in diameter than usual, which isn’t noticeable to your eye — and I don’t recommend trying to find out — but is pretty obvious in photographs using telescopes and heavy filtering, like this one from astrophotographer Anthony Ayiomamitis:

Cool, huh? When we’re farther from the Sun we receive a bit less heat, so perhaps those of you suffering from the midwest heat wave can take consolation that it could be worse by a couple of degrees right now.

Later today, coincidentally, I’ll be at a picnic with lots of solar astronomers. What do I say to them? "Hap-helion Fourth of July"? Or, "Enjoy us being at a(1+e) [where a = 1 AU and e = 0.0167] from the Sun today"?

That seems awkward. The thing is, I’m pretty sure a lot of them would get it…


Related posts:

At the bottom of Earth’s orbit
Happy New Year again!
Why we have leap days
Does this perihelion make my Sun look fat?
Does the Sun look smaller to you?


* I pronounce it app-HEEL-eeyun, if you care.

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

Comments (52)

  1. Nigel Depledge

    Happy 4th July, BA!

    And aphelion, too. No, I don’t have any clever suggestions about how to greet astronomers on a day like today.

    Interestingly, the final signature was applied to the Declaration of Independence on the 3rd of July. But, in the days before photocopiers, documents had to be copied out by hand for circulation. The date on the bulk of the copies of the Declaration was the 4th of July, and this is what stuck. (Source: Bill Bryson, Made In America).

  2. A depressing number of people in the UK actually think summer and winter are caused by the Earth being closer to and further from the Sun!!! I guess they are too stupid to even know that the seasons are reversed in the Southern Hemisphere. Some are astonished, when we tell them that we are actually closest to the Sun in January.
    Last December, when the UK was experiencing some exceptionally cold weather, some imbecile of a so-called journalist, writing on my ISP’s “news” pages, commented: “It might get even worse on Tuesday, with the Winter Solstice, the time when the Earth is furthest from the Sun.”
    I kid you not!!!!!!
    I have no idea what they teach kids in school these days. When I was a kid, we learned about the tilt of the Earth’s axis and the cause of the seasons at the age of 12.

  3. Electro

    Looking more like 3.3% difference to me….can anyone explain where I am going wrong here?

  4. Makoto

    Also reminds me of posts like:
    “** Fact – if the earth was 10 ft closer to the sun we would all burn up and if it was 10 ft further away we would freeze to death… God is amazing!”

    Less than a 20 ft habitable zone, an extremely comfortable 3-ish million mile (to stay in their units) habitable zone, what’s the difference?

  5. gdave

    @Neil Haggarth:

    A lot of folks in U.S. (and, I’d imagine, most everywhere in the world) think the same thing. That doesn’t make them stupid, though. And I’d bet they still teach axial tilt as the cause of the seasons to kids in school.

    Sherlock Holmes famously revealed, in one story, that he didn’t know the Earth orbited the Sun. When pressed by Dr. Watson, he replied that his brain was like an attic – he had to throw out irrelevant facts to make room for the facts he needed to solve crimes. A bit more extreme, but a pretty good analogy.

    The cause of the seasons is simply irrelevant to the lives of the vast majority of people, and not a fact they encounter often, if at all, after grade school science class. People tend to retain the facts that are useful or important to them, or which they frequently encounter. Thinking that the seasons are caused by distance to the Sun at least indicates that they realize that the Earth’s orbit is elliptical.

    I’d be happier if more people understood the cause of the seasons better – but I’m not depressed that they don’t. I think a solid understanding of basic statistics, probability, and the scientific method are much more important for the average person, and I get a bit depressed sometimes that those are so widely misunderstood.

  6. Axial tilt is the reason for the season (kind of feels like I’m doing some sort of christmas in July thing). ;)

  7. Larry

    “I’ll be at a picnic with lots of solar astronomers”

    Careful, there, BA. I hear them guys ‘n gals are Par-tay Animals! And don’t even think about answering their knock-knock jokes.

  8. Sir Chaos

    So if aphelion is in Northern hemisphere summer, and perihelion in Northern hemisphere winter, whereas in the Southern hemisphere it´s the other way around, wouldn´t that mean that, all other things being equal, seasonal temperature variations are slightly more extreme in the Southern hemisphere compared to the Northern hemisphere?

  9. chris j.

    When we’re farther from the Sun we receive a bit less heat, so perhaps those of you suffering from the midwest heat wave can take consolation that it could be worse by a couple of degrees right now.

    actually, no. while we receive a little less solar radiation, including heat, at aphelion, the difference is completely subsumed by atmospheric conditions and other local factors such as land/water ratios, oceanic currents, and geography.

  10. Electro,

    Phil is referring to the deviation from the average distance between the Earth and the Sun. However, relative to aphelion, it is indeed approximately twice that figure.

    Anthony.

  11. hhEb09'1

    Phil says the 0.24 (actually 0.26) extra days per revolution is the main cause of the difference from year-to-year but last year the aphelion was on July 6 at 12:00 UT, compared to this year July 4 at 15:00 UT, a difference of almost two full days. Luckily, this is rocket science.

  12. Jason

    @Neil and gdave: Funny this comes up. I am a 4th grade math and science teacher and I assure you we do teach that it is the tilt of the axis and the resulting direct/indirect light rays that cause the seasons. However, I have recently been doing research on misconceptions (or, if you want to be nicer “alternative conceptions”). Turns out people will hold on to their misconceptions with way more determination than most of us think. The best way for someone to overcome a misconception is to have an experience that shows it is wrong. Most people wont change their mind simply because they are told it is wrong. However, even in the face of an experience people will hold on to and justify their currently held beliefs. I saw a video of 7th graders that believed if they were in a totally dark room (totally dark, no light at all) that they would eventually be able to see an apple sitting on a table because their eyes would adjust. To get them to understand their error they actually did it. Even after 5 minutes of not being able to see the apple they still kept saying, “maybe a little longer.” One girl said, “it may take a couple of years, but I still think our eyes will adjust.”

  13. mike burkharrt

    #5 gdave I would not say the seasons are irelovent to all people ,you should hear my brother all winter when his car gets stuck in the snow. I think to many people it seems reasonable to think that the seasons are caused by the distance of the Earth to the Sun ,but as all Astronomers know things in the Universe are not allways as they appere.To quote transfomers ”more then meets the eye”

  14. Jason

    Here is the video. The apple in the dark room starts at the 30 minute mark…

    http://www.hsdvl.org/video.php?record_serial=151

  15. Historical irony – using cathedrals as giant pinhole cameras, Jesuit astronomers as part of calendar reform discovered the eccentricity of the Earth’s orbit for themselves a generation after Galileo, and while they were at it confirmed that Kepler rather than Ptolemy was right (see J.L. Heilbron’s book “The Sun in the Church”). I still try to imagine them deciding who got to tell the pope.

    And Jason – find a copy of the video “A Private Universe” – Harvard grads show the same behavior, even after taking the relevant classes. At least that knowledge can help us try to head this off a bit in the classroom.

  16. Nigel Depledge

    Gdave (5) said:

    When pressed by Dr. Watson, he replied that his brain was like an attic – he had to throw out irrelevant facts to make room for the facts he needed to solve crimes. A bit more extreme, but a pretty good analogy.

    Unfortunately, there is no evidence to suggest that this is anything like what actually happens. We carry on accumulating knowledge as we grow older.

    Having said that, of course most kids would not retain the reason for the seasons. But not because they are making space for new information, rather because they never cared to start with.

  17. Cindy

    Happy aphelion to you, Phil! And Happy 4th of July!

    Actually the Declaration of Independence was agreed upon on July 2nd and John Adams wrote to Abigail Adams expecting that the 2nd of July would be a holiday in future years. I remember hearing that from a docent at the John Adams house in Quincy, MA (on July 2nd, no less) and also remember reading it in a biography of John Adams.

    (Was sort of looking forward to your usual Declaration of Independence posting, but this is just as interesting).

  18. GebradenKip

    Sorry to be nitpicky, but this is actually not true:

    If you’re curious, that’s about 1.67% farther from the Sun than on average. That in turn means the Sun appears about 1.67% smaller in diameter than usual, which isn’t noticeable to your eye

    1.67 % increase in distance for the sun means “only” a 1.64 % decrease in angle (and thus apparent diameter). Trigonometry, it works b*tches!

  19. Chris Browet

    Call me dumb, but could someone explain me in not too complicated words why the aphelion and the summer solstice do not happen at the same time?

    Thanks

  20. i teach 7th grade science and cover the reason for the seasons being the tilt of the earth. there are a lot of raging hormones in the room, so these kids are naturally distracted and not listening very closely sometimes; so it was not at all surprising to hear this comment: “23.5 degrees? that’s pretty cold to me!” hopefully, i corrected their misconception, but it sure is hard to keep a straight face.

  21. Melissa

    Forgive me, I know it’s only tangentially related, but this little cartoon made me laugh: http://thedailyfunnies.files.wordpress.com/2009/06/06-26-09-mutts.gif?w=450&h=141

  22. Dan

    @19 Chris Browet:

    It’s because the seasons are caused by the Earth’s tilt, not its distance from the Sun. When the hemisphere you’re on is tilted towards the Sun, you experience more day than night, which warms up that side of the planet (Summer). When the hemisphere you’re on is tilted away from the Sun, you experience more night than day, which cools that side of the planet (Winter). The effects of perihelion and aphelion are very small compared to the effects of the tilt.

  23. Marina Stern

    Phil, if you really want to freak out your buddies, pretend that you’re not aware that today is Independence Day, and make out that the reason for the picnic and fireworks is solely the aphelion. If you can keep up the pretense for a whole five minutes, I’ll front you some sausages from Andy’s Deli.

  24. Robin Byron

    “* I pronounce it app-HEEL-eeyun, if you care.”

    Where are our favorite pedants today? ;)

  25. @Sir Chaos (#8), not that I am aware of. There are many factors that would play into that, one of the largest being the difference in the surface area of land versus water in the southern hemisphere. I haven’t looked into some of the variations that are recorded specifically for the southern hemisphere, but that would make for some interesting research.

  26. Chris Browet

    @Dan

    Thanks.

    I knew seasons were due to the tilt, I somewhat assumed the tilt was also “at its max” (meaning sun rays were shortest/longest) at the aphelion.

    Thinking about it, it is quite a stupid assumption ;-)

  27. Mike

    @Chris

    Right now the southern hemisphere is the right way ’round with winter at aphelion. However the Earth precesses (the north pole wobbles around the perpendicular to our orbit once every 26,000 years) so the date of the aphelion will change. It also turns out that the aphelion “location” relative to fixed stars changes too, and goes around once every 113,000 years. According to Wikipedia (because I’m too lazy to do the math myself), the combined factors means the date of aphelion goes around once every 21,000 years. If you wait about half that, or about 10 thousand years, the distance from the sun will line up with the seasons here in the north.

  28. Pete Jackson

    @8, 18: The Earth is 1.67% farther from the sun at aphelion and 1.67% closer at perihelion. That is a 3.34% difference between the nearest and farthest distances. The incident energy received from the Sun varies as the inverse square of the distance, so we get twice,that difference or 6.68% more energy at perihelion than at aphelion. The equilibrium temperature of the Earth (assuming a black-body radiator with no atmosphere; the Moon gives a good approximation) varies as the fourth root of the energy received, so the equilibrium temperature (on the absolute Kelvin scale) will be 1.67% greater at perihelion that at aphelion. (All this switching between powers and ratios is just elementary calculus).

    Assuming a mean equilibrium temperature of 288 degrees Kelvin, we derive that the equilibrium temperature at perihelion would be 0.835% greater, or about 2.4 degrees higher at perihelion, or 290.4 degrees Kelvin and 2.4 degrees lower at aphelion, or 285.6 degrees Kelvin. These figures are 17.1 and 12.3 degrees Celsius respectively and 62.8 and 54.1 degrees Fahrenheit. So if aphelion occurred in winter and perihelion in summer, we who live in the northern hemisphere might expect summers eight degrees Fahrenheit warmer than they are now, and winters eight degrees Fahrenheit colder than they are now!

    Fortunately, the southern hemisphere temperatures are moderated more the oceans than in the northern hemisphere, so the greater extremes that you would expect don’t normally show. But Australian summers can get sure darn hot at times!

  29. Sir Chaos

    Thanks for the number-crunching, Pete.

    Australian summers may get damn hot, but Australia is closer to the equator than a lot of the inhabited Northern hemisphere. I mean, I´m at 51° North here (Germany), and we get 2-3 weeks of 100°F or so each summer.

  30. Sam

    @Mike
    Assuming the definition of a year is the time it takes the earth to do one orbit of the sun, does this mean that (due to precession) that the seasons gradually shift relative to 1 Jan every year? So in 10,000 years or so, christmas will be in the middle of summer for you northerners, and the middle of winter for Australians?
    I guess it depends on our definition of a year… I may have found the answer to my own question here http://en.wikipedia.org/wiki/Sidereal_year : “The difference is caused by the precession of the equinoxes, and means that over long periods of time a calendar based on the sidereal year will drift out of sync with the seasons at the rate of about one day every 72 years.”

  31. I wouldn’t mind sending a little rain down to the lower-48, before we drowned up here. What a crazy spring and summer…

  32. Brian

    Dammit, Phil, I cannot explain to coworkers at my retail job why a(1+e) is funny when they ask what I’m laughing about in the breakroom!

  33. JB of Brisbane

    Once when I was at the Tourist Information Centre in Rockhampton, central Queensland (which displays an incorrectly-placed Tropic of Capricorn marker out the front), I tried to explain to a young French tourist what the Tropics were all about. I don’t know if it was her lack of geographical and astronomical knowledge, or the limit of her grasp of English, or both, but she just couldn’t comprehend the whole “tilt of the Earth’s axis” thing. I thought I was explaining it pretty simply, but I was getting nowhere fast. I was going to see if the staff had a globe and a powerful torch (flashlight) to demonstrate it.
    BTW – the Tropic of Capricorn actually passes about 3km south of where the marker is now.

  34. Happy Independence Day to my US friends. :-)

    (Belatedly, sorry, RL intruding.)

    & a Happy Aphelion to all! :-)

  35. @6. Larian LeQuella : “Axial tilt is the reason for the season (kind of feels like I’m doing some sort of christmas in July thing).”

    Well, Christmas was originally a solstice festival that got syntheisised and reworked into the modern day event (hence the Yule log, mistletoe & holly, pine tree, feasting & drinking, etc..) – and it *is* around the solstice give or take a week so, yeah. ;-)

    @4. Makoto :

    Also reminds me of posts like : “** Fact – if the earth was 10 ft closer to the sun we would all burn up and if it was 10 ft further away we would freeze to death… God is amazing!”

    Was that discussed on the BA blog or Pharnygula, hmm .. let’s see :

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

    My quick google search :

    http://www.google.com/search?q=%E2%80%9C**+Fact+%E2%80%93+if+the+earth+was+10+ft+closer+to+the+sun+we+would+all+burn+up+and+if+it+was+10+ft+further+away+we+would+freeze+to+death%E2%80%A6+God+is+amazing!+BA+blog,+Pharnygula&hl=en&biw=1600&bih=732&gbv=2&source=lnms&ei=rL0STpeSPMnvmAX_xLjIDg&sa=X&oi=mode_link&ct=mode&cd=1&ved=0CCkQ_AUoAA#sclient=psy&hl=en&gbv=2&source=hp&q=BA+Blog+%E2%80%9CFact+%E2%80%93+if+the+earth+was+10+ft+closer+to+the+sun+we+would+all+burn+up+and+if+it+was+10+ft+further+away+we+would+freeze+to+death%E2%80%A6+God+is+amazing!&aq=&aqi=&aql=&oq=&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=bb9a4fba297be287&biw=1600&bih=732

    says Pharnygula – with this post high on the list.

    Either way, its very much false – not only is the Earth’s orbit elliptical and also the Sun has shifted in luminosity growing steadily brighter over its billion years of man-sequence life so far but the Solar Habitable Zone (HZ) :

    http://en.wikipedia.org/wiki/Habitable_zone

    I think I recall reading / hearing somewhere that our solar HZ ould arguably extend out as far as Mars with the red planet’s issue being its low mass, lack of magnetic field and the consequent erosion of it’s atmosphere by the solar wind. Or so I gather, could be mistaken of course .

    Plus – d’uh! – mountains and hills higher than 10 ft of which there are plenty – all of them really! ;-)

    @ 1. Nigel Depledge :

    Interestingly, the final signature was applied to the Declaration of Independence on the 3rd of July. But, in the days before photocopiers, documents had to be copied out by hand for circulation. The date on the bulk of the copies of the Declaration was the 4th of July, and this is what stuck. (Source: Made In America, Bill Bryson, ).

    Cheers! Didn’t know that. :-)

  36. Nigel Depledge

    Chris Browet (19) said:

    Call me dumb, but could someone explain me in not too complicated words why the aphelion and the summer solstice do not happen at the same time?

    Although someone (#22) addressed this, I don’t think that was a complete answer to this question.

    The reason they don’t happen at the same time is because they are not connected.

    The seasons arise because of the Earth’s axial tilt. At the moment, the axis of Earth’s rotation is roughly pointing at Polaris in the northern sky and a sort-of blank patch (in or near Crux, IIRC) in the southern sky.

    This is a separate thing from aphelion and perihelion, which are phenomena connected to the shape of Earth’s orbit as it revolves about the sun (or, more accurately, about the solar system’s barycentre). There is no reason why the axial tilt of the Earth should line up with the long axis of its elliptical orbit, and we find that it does not line up.

    If they did line up, then we would have aphelion and perihelion at the solstices.

  37. Messier Tidy Upper

    “Enjoy us being at a(1+e) [where a = 1 AU and e = 0.0167] from the Sun today”?

    How would you actually pronounce – verbally say – that eh? Have fun trying! ;-)

    @4. Makoto :

    Also reminds me of posts like : “ Fact – if the earth was 10 ft closer to the sun we would all burn up and if it was 10 ft further away we would freeze to death… God is amazing!” Less than a 20 ft habitable zone, an extremely comfortable 3-ish million mile (to stay in their units) habitable zone, what’s the difference?

    So the tallest humans – the basketballers and circus giants – that are 7 or 8 feet high are
    going to be in a lot of trouble when they jump then? Oh & mountaineers and divers. What .. The ..!!! *Facepalm* :-o ;-)

  38. Nigel Depledge

    Mike (27) said:

    Right now the southern hemisphere is the right way ’round with winter at aphelion. However the Earth precesses (the north pole wobbles around the perpendicular to our orbit once every 26,000 years) so the date of the aphelion will change. It also turns out that the aphelion “location” relative to fixed stars changes too, and goes around once every 113,000 years.

    Erm … I’m not so sure this is right.

    Yes, the Earth precesses on its axis, so the dates of the equinoxes and solstices change, but this won’t change the date of aphelion.

    However, as you point out, the orientation of Earth’s orbit is also changing over time (due perhaps to gravitational interactions with other planets?), so the dates of aphelion and perihelion will change, but this is over a different cycle from the precession of the axis. (In essence, a date can be viewed as a coordinate for a position of the Earth on its orbit around the solar system’s barycentre).

    According to Wikipedia (because I’m too lazy to do the math myself), the combined factors means the date of aphelion goes around once every 21,000 years. If you wait about half that, or about 10 thousand years, the distance from the sun will line up with the seasons here in the north.

    Yet according to Wikipedia, this 22,000 to 26,000 year variation (Not 113,000: which page did you look up? I got these figures from the page referenced as “Apsis”) is purely due to the precession of Earth’s orbit, and nothing to do with the precession of the Earth’s axis.

    Specifically, Wikipedia says “On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth that is called the precession of the orbit. (This is not the precession of the axis.)”

    So, to summarise, there are two separate effects that you seem to have tangled together here:

    The first is the precession of the Earth’s orbit (that is, the major axis – also known as the line of apsides – of the Earth’s orbit turns over this cycle of 22,000 to 26,000 years) relative to the background stars. This causes the dates of aphelion and perihelion to change.

    The second is the precession of the Earth’s axis, which causes the dates of the solstices and equinoxes (and hence also the timing of the seasons) to change, and also causes the position of the celestial poles to change.

  39. Mike

    @Sam

    The calendar tries to stay in sync with the seasons; that’s why we built a calendar in the first place. It’s timed from equinox to equinox, making sure that it takes precession into account. However, precession causes the signs in the zodiac to shift, so that Pisces in the beginning of March isn’t actually true anymore. Astrology is more than 2200 years old, which means it’s just over one constellation off by now. Somehow that doesn’t seem to bother astrologers.

  40. #5 gdave, #16 Nigel:
    I think Nigel is right; people don’t “throw away irrelevant facts” because they are making room for other information, but more likely because they just don’t care. Like those people who go abroad on holiday, and put their watches forward or back, because the cabin crew tell them to, but neither know nor care why! ( Don’t they even know that the Earth rotates? )
    I’ve told the story before, about the idiot who couldn’t figure out which direction was north or south, at 6 p.m. on a summer day. I find it very hard to believe that anyone could not know that the Sun rises in the east and sets in the west – but presumably they just regard it as an “irrelevant” fact, which they learned at school and then forgot about, and it never occurs to them that such a fact can actually have a practical application in life, such as finding directions. Or to put it another way, they are not capable of applying simple logic, to use known facts to deduce an unknown one. To me, that does make them stupid!
    Sorry, but I just can’t comprehend this “who cares” mentality. Nor can I understand how “forgetting” something which was taught at school, such as the cause of the seasons, can result in people believing something completely false.

    #12 Jason:
    “Alternative conceptions” – no, “misconceptions” is right! We’re talking about cases where there is no “alternative”; there is just correct and wrong.

  41. Mike

    @Nigel

    The simplest description on Wikipedia is here:
    http://en.wikipedia.org/wiki/Precession#Astronomy

    The first block describes the 26,000 year wobble of the Earth’s axis as axial precession. The second block describes a variation in the 23 degree tilt up and down, which isn’t relevant here. The third section indicates the change in the location of aphelion/perihelion from year to year, and contains a link to http://en.wikipedia.org/wiki/Apsidal_precession where I got the 112,000 year cycle (somehow I miscopied that as 113,000) and the combined value of 21,000 years.

    “22,000 to 26,000 years” is kind of a big range, and may just be a confusion of the difference between the axial precession itself and the axial precession combined with the apsidal precession.

  42. Jesse

    And now for something completely different.

    When greeting your fellow astronomers today, use the old hippy standby:
    “Far out!”

  43. Makoto

    37. Messier Tidy Upper:
    “So the tallest humans – the basketballers and circus giants – that are 7 or 8 feet high are
    going to be in a lot of trouble when they jump then? Oh & mountaineers and divers. What .. The ..!!! *Facepalm*”

    Exactly.. exactly. It really makes one wonder about people sometimes, doesn’t it?

  44. Nigel Depledge

    @ Mike (42) –
    Yes, it looks like you’re right about those numbers. The wikipedia page I found seems to have got its figures confused.

    However, that does not change that your earlier post (27) seemed to conflate two of the different types of precession. I refer in particular to this sentence:

    However the Earth precesses (the north pole wobbles around the perpendicular to our orbit once every 26,000 years) so the date of the aphelion will change.

    This is what I initially thought wasn’t right. Then I found the wiki page on Apsis and got a different set of numbers from yours as well.

  45. 39. Mike Says:
    July 5th, 2011 at 5:33 am
    Astrology is more than 2200 years old, which means it’s just over one constellation off by now. Somehow that doesn’t seem to bother astrologers.
    ——
    Apparently you missed the horoscopes in The Onion from earlier this year when Ophiuchus was being discussed as the “13th sign”: http://www.theonion.com/articles/your-horoscopes-week-of-january-18-2011,18855/

  46. Scarlet

    @Neil Haggath: Yes, that is rather disheartening! Just as sad is an experience I once had in an introductory college astronomy class. After completing the unit on the Sun and discussing the telescopes on campus, one student inquired, “So, how do you take pictures of it? At night, when it’s far away?”

  47. hhEb09'1

    @Nigel
    That sentence is exactly correct, by itself. If there were no other change but precession, the dates would change, because the precession has caused us to use a calendar that is not tied to the stars.

  48. #47 Scarlet:
    Please, please tell me that was a joke…!

    In a recent UK survey into education standards, one of the “general knowledge” questions which a sample of adults were asked was, “Does the Earth go around the Sun, or the Sun around the Earth?” The number who answered correctly was… wait for this… 67%!!!!! 19% actually got it wrong, and 14% said they didn’t know!
    Head, meet wall.

  49. Finlay

    >>>I pronounce it app-HEEL-eeyun, if you care.
    I recently read the wikipedia article that said it should be pronounced like afelion. I now feel less alone for having always pronounced it ap-helion like you. Hooray!

  50. #50 Finlay et al:
    “App-helion” is correct. The P and H are definitely not pronounced as an F; they are two separate consonants, belonging to two separate syllables. The “ap” is a separate syllable, as is obvious if you consider the composition of the word.
    The opposite term, for the closest point in the Earth’s orbit, is “perihelion”. “Helion” comes from Helios, the Greek word for the Sun, “peri” and “ap” are prefixes appended to it. Similarly, the closest and furthest points of the Moon’s orbit around the Earth are called perigee and apogee; for the orbit of a satellite of Jupiter, the words are perijove and apojove; for the orbit of the secondary around the primary in a binary star system, they are periastron and apastron. There are even a pair of generic words – periapsis and apoapsis – which can be applied to any orbit.
    So we see that “ap” is a prefix appended to a whole range of words; so the pronunciation is “app-helion”. QED.

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