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.

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.

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

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

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.

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

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.

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.

“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*

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.

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.

(Belatedly, sorry, RL intruding.)

& a Happy Aphelion to all!

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.

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!