Today, January 3, on or about 20:00 Universal Time (2:00 p.m. Pacific time), the Earth will reach perihelion, its closest approach to the Sun. The distance from the Sun to the Earth will be roughly 147,093,600 kilometers (I have found several different distances on different sites, and this is an eyeball average).
Over the course of the year, the Earth’s distance from the Sun changes because the orbit is not a perfect circle. It’s an ellipse. The average distance of the Earth from the Sun (what astronomers call an astronomical unit) is 149,597,871 kilometers. So you can see that we will be 2.5 million kilometers closer to the Sun than average at 20:00 today, a difference of about 1.6% or so. If you had a good ’scope and measured the Sun very carefully today, then measured it again on July 7, when we reach aphelion, the farthest distance from the Sun, you’d see it would be roughly 3% bigger today.
Remember, our distance from the Sun doesn’t affect our seasons (much). Because the heat we receive from the Sun depends on the square of our distance, we get 6% more heat today than we do in July. Northern hemisphere denizens may be surprised by this, but there you go. Astronomy is many things, and surprising is one of ‘em.
Also, because we’re closer to the Sun, we feel the effects of its gravity more. This doesn’t affect us all that much, except that tides from the Sun will be stronger now than in July. It’s not a big effect, since the Moon’s tides are twice as strong as the Sun’s to start with. But again, if you took very careful measurements of water heights, you could detect this.
I’m sure many of my readers already know this stuff to some extent. But if you didn’t, then I suppose for you there is something new under the Sun. And that’s one of my favorite things in the whole world… and off of it, too.
Happy perihelion!
January 3rd, 2007 at 1:03 am
Oh no! We’re going to hit the sun! Look out! Waaaah!
January 3rd, 2007 at 2:52 am
What is Universal Time??
Is it the same as GMT?
January 3rd, 2007 at 4:24 am
Many thanks go to Johannes and Brahe. It took Kepler eight years to find an elegant model that fit the data of planetary positions meticulously accumulated by Brahe.
We would normally expect that the northern hemisphere would have slightly more pronounced seasons because the perihelion closely coincides with the northern summer solstice (and aphelion with the northern winter solstice). However, the southern hemisphere has a lot more surface water to moderate climate. At least that was what an astronomer giving a lecture once said.
January 3rd, 2007 at 4:45 am
20:00 UT is more like noon Pacific I think
January 3rd, 2007 at 5:17 am
Tim.
Perihelion is the closest approach.
I would expect the northern hemisphere to have *less* pronounced seasons because the Sun is closer in winter and farther away in the summer. This would tend to decrease the effects of the seasons there.
On the other hand the southern hemisphere should have *more* pronounced seasons because the Sun is farther away in their winter and closer in their summer. This would tend to increase the effects of the seasons there.
January 3rd, 2007 at 5:42 am
Michael H,
Oops, typo.
January 3rd, 2007 at 5:59 am
So it’s the sun that’s playing havoc with my scale. I’m relieved it wasn’t all the goodies for the last month. Now BA could you please come up with a scientific explanation (other than too many calories) as to why my pants get tighter around this time of the year.
January 3rd, 2007 at 6:32 am
As Homer Simpson pointed out – “The sun??? That’s the hottest place on earth!”
January 3rd, 2007 at 6:50 am
I thought it felt rather warm today.
I wonder if there are some that try to see if the sun triggers earthquakes and volcanic eruptions, just as they do with the moon. I wouldn’t put it past them.
Anyway, happy perihelion day to all!
January 3rd, 2007 at 6:51 am
“What is Universal Time??
Is it the same as GMT?”
For all practical purposes, yes!
January 3rd, 2007 at 7:14 am
Would that be why it has seemed like the sun is brighter here lately, or is that one of those logical fallacies that good scientists warn us against?
January 3rd, 2007 at 9:25 am
The difference between GMT and UTC, if I recall, is that UTC never has daylight savings time, but the GMT time zone does, so at some times of the year GMT = UTC+1
January 3rd, 2007 at 9:31 am
If you’re curious, here’s some text from the Wikipedia on the difference between GMT and Universal Time (and mind you, Universal Time has several flavors):
“In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. However, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.”
“There are several versions of Universal Time:
UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. It is uncorrected for the displacement of Earth’s geographic pole from its rotational pole. This displacement, called polar motion, causes the geographic position of any place on Earth to vary by several metres, and different observatories will find a different value for UT0 at the same moment. It is thus not, strictly speaking, Universal.”
“UT1 is the principal form of Universal Time. It is computed from the raw observed UT0 by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1 is the same everywhere on Earth, and is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference.”
“UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based.”
So strictly speaking, when somebody talks about such-and-such a thing happening at 20:00 Universal Time, it’s UTC that they’re talking about. For most folks, this is the same as GMT, and is sometimes also called Zulu time (which refers to the nautical time zone containing the prime meridian).
Lorne
http://geekcounterpoint.net
January 3rd, 2007 at 9:45 am
As it happens, this perihelion is also near the Full Moon, which will amplify the (small) effect that perihelion will have on the tides.
Lorne, is the “polar motion” you quote simply small-scale wobble about the pole? IIRC, that’s the effect of geologic-scale movements of mass, both crust plates and deeper magma flows, right?
January 3rd, 2007 at 10:01 am
Thanks for the reminder. To celebrate, we plan to have a traditional Perihelion Day dinner of Wienerschnitzel chili cheese dogs. The wine selection is still TBD, but I’m leaning toward a Nickel and Nickel single vineyard cab.
January 3rd, 2007 at 11:08 am
“The difference between GMT and UTC, if I recall, is that UTC never has daylight savings time, but the GMT time zone does,”
No, no, no! GMT does not have time zones (except in some rather misleading Microsoft software). Some countries (e.g., at the western end of North Africa) use GMT all year. Other countries (e..g, the United Kingdom) use GMT in the winter and in summer use GMT+1 called whatever locally (BST – British Summer Time, in the UK). GMT itself stays put all year round.
January 3rd, 2007 at 11:30 am
Time is hard!
January 3rd, 2007 at 1:06 pm
Does anybody really know what time it is?
January 3rd, 2007 at 2:03 pm
I should point out that you don’t need instruments that are all that careful to see the tidal effects of being closer to the Sun. I was in Santa Barbara on the 1st (I hear the cries of pity coming in already) and had a chance to enjoy the extreme low tide they were experiencing that afternoon. In general, the spring tides (in other words, the tides that occur at new and full moons) in January are the most extreme of the year because of this.
January 3rd, 2007 at 2:28 pm
Time is hard!
No kidding. All that’s left is for somebody to bring up Special Relativity and the fact that there is no universal “now”.
D’oh!
January 3rd, 2007 at 4:12 pm
[...] Phil Plait wünscht uns allen ein schönes Perihel. Ich schließe mich, nachdem ich mich grade über die Nichtverschiebbarkeit von Silvester beschwert habe, an – feiert schön! [...]
January 3rd, 2007 at 8:42 pm
Yup, time is hard! UTC is 8 hours ahead of PST, so 2000 UT is 1200 PST is
noon California time, not 2PM! So we missed it. I always thought the
funnest part of a roller coaster ride was at the top of the hills, just as you
start teetering, not the valleys, which you zoom by so fast you miss them, so everyone wait for July!
January 4th, 2007 at 7:12 am
“you’d see it would be roughly 3% bigger today”
So the sun gets bigger in the winter? Where does the extra mass come from – or does the density go down?
tsk-tsk-tsk I though this site was about correcting misconceptions.
January 4th, 2007 at 7:41 am
Phil,
If I remember correctly, you took German in school. If jstarek.de cannot or will not write in English, then how about translating his comments. I would like to know what he/she has to say. [Of course, I am assuming it is German.]
Or maybe one of your bloggers can help out.
BA Mom
January 5th, 2007 at 2:04 pm
Somewhat related, on 17 Dec 03, while perusing the National Weather Service’s web site, I came upon this weather warning for Turney, MO:
.UNUSUALLY HOT WEATHER HAS ENTERED THE REGION FOR DECEMBER…AS THE EARTH HAS LEFT ITS ORBIT AND IS HURLING TOWARDS THE SUN
The top of the statement was labeled “..TEST…TEST…TEST…”, and its authorship was attributed to “HEINLEIN”. But obviously somebody didn’t have a sense of humor: the warning was gone when I tried to call it up again to show coworkers. Fortunately, I had printed it out, and it’s now in my Funny File
January 8th, 2007 at 10:57 am
Babelfish offers this translation of jstarek.de:
“Phil Plait wishes us everything a beautiful perihel. I follow, after I weighted degrees about the Nichtverschiebbarkeit of Silvester, – celebrates beautifully!”
Hmmm, well that sure helps, eh?
I know that Phil gave a rather precise distance for perihelion and I find it interesting that the value was not simple to determine. The internet, of course, offers many opinions, but this type of measurement should be known and published (as by NASA) as an accurate number. With such precision, by the way, is the distance between the Earth and Sun from surface to surface? Or center to center? Only curiosity makes me ask as I don’t think it will affect me much either way….
Also, does the speed of the Earth vary much from perihelion to aphelion? Do we speed up as we descend in orbit with a gradual slowing as we move further out? Or is it the opposite, so that we maintain roughly the same progress in degrees as we move to a longer orbital arc?
Either way, the roller coaster analogy does clearly explain my continuing dizziness and light-headed feeling that I experience every year around this time!!
January 17th, 2007 at 3:31 pm
[...] Thanks to Bad Astronomy for the reminder. This entry is filed under Space. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site. Leave a Reply [...]
January 18th, 2007 at 2:52 pm
Evelyn, I just noticed it doesn’t make much sense to send a trackback to an English blog if your own posts are in German, sorry!
I didn’t write anything interesting, though, just a pointer to this nice article. It translates to “Phil Plait wishes us all a happy perihelion. I’d like to join him, especially now that [a few posts earlier] I complained about the fact that New Year’s eve can’t be moved to anytime you like”.
January 25th, 2007 at 4:10 am
[...] Happy Perihelion everyone! (via Blog Physica and Bad Astronomy): Today, January 3, on or about 20:00 Universal Time (2:00 p.m. Pacific time), the Earth will reach perihelion, its closest approach to the Sun. The distance from the Sun to the Earth will be roughly 147,093,600 kilometers (I have found several different distances on different sites, and this is an eyeball average). Over the course of the year, the Earth’s distance from the Sun changes because the orbit is not a perfect circle. It’s an ellipse. The average distance of the Earth from the Sun (what astronomers call an astronomical unit) is 149,597,871 kilometers. So you can see that we will be 2.5 million kilometers closer to the Sun than average at 20:00 today, a difference of about 1.6% or so. If you had a good ’scope and measured the Sun very carefully today, then measured it again on July 7, when we reach aphelion, the farthest distance from the Sun, you’d see it would be roughly 3% bigger today. [...]
January 22nd, 2008 at 9:13 am
I think Phil meant “… would APPEAR to be 3% bigger …).
It’s all about appearances, man.
Ivan.
PS, I’m back, reading the few items left over from early last year, that I missed while on holidays. Shame is, I also have to admit that I didn’t read any from April 2007 onwards, until last night. I’m working through them now, and slowly into the future, while also reading the latest from now.
I think it is going to take a while to catch-up.
Ivan.