It is 12:18:18 on January 1st, 2007. Happy Sidereal New Year! 12:18:18? Sidereal? Wha..?

Sidereal refers to the stars. It exactly 365.25636051 days long (365 d 6 h 9 min 9 s). There are many different kinds of years, different ways of measuring time by the positions of the Earth, Sun, and stars, and you can read about them here in Wikipedia. But I’m just going to briefly talk about the sidereal and tropical years.

Imagine drawing a line from the sun, through the Earth, and straight to some star in the great distances of space. The time it takes for the Earth to orbit around the sun, and come back to that line is one Sidereal Year. You could do the same for a day - the time it takes for the Earth to rotate around so that the same star is in the same position in the sky is one sidereal day.

A sidereal day is sbout 4 minutes shorter than a solar day, 236 seconds to be more exact, which adds up to one more sidereal day in the year than a solar day. Why is that? Let me put it to you this way. If the Earth didn’t rotate at all, every solar day would be as long as a year. If the Earth rotated once per orbit, then the same side would always face the sun and you would perpetual light and dark on each side of the Earth, kind of like one side of the moon always faces the Earth. Because Venus rotates so slowly, the Venusian solar day is actually longer than the Venusian year!

Keep in mind this calculation works when the planet rotates the same direction it orbits, or the prograde direction. If the rotation of the planet were in the reverse or retrograde direction. All bets are off in defining “days” if you’re on a floating on Uranus, which has its axis tilited all the way over.

Anyway, you can easily see that rotating around the sun subtracts one solar day from the number of times the Earth actually rotates.

Back to the issue of years. There is another major concept called the Tropical Year(or Solar Year), which is less absolute in position than the sidereal year. The tropical year refers to the position of the sun relative to the Earth’s equator. Or technically, the intersection of the ecliptic (plane of Earth’s orbit) with the Equator. Whosaywha?

As you know, Earth is tilted to the side, which gives us our equinoxes, solstices, and the variability in light and warmth that gives us our seasons. Because the tilt point one direction, when the Earth is on different sides of the sun, the north and south hemispheres get exposed to different amounts of light, and have longer and shorter days. The path of the sun’s position at noon (or the ecliptic, if you will) each day snakes its way up from the equator at the equinox, to the Tropic of Capricorn for the summer solstice (Northern Hemisphere, here!), and back down through the equator again for the autumnal equinox. The winter solstice happens when it goes all the way down to the Tropic of Cancer, and then back up again to the euator for the equinox in the spring.

You can measure the length of the Tropical Year, or the time it takes to cycle through the seasons, by measuring the time between two Equinoxes a year apart. If you measure the time from Equinox to Equinox, however, you get 365.24218967 days (365 d 5 h 48 min 45 s). But wait, why are the two years different lengths? I thought we were talking about years - shouldn’t they be the same no matter how we measure it?

From year to year, the size of the Earth’s orbit and the length of a year is almost exactly the same. However, because of something called precession, the tropical year and sidereal years do not match up. The Earth’s tilt actually rotates on a 26,000 year cycle - think of it like a top or a gyroscope spinning while tilted. Or if you have a tilted globe, you can visualize it by rotating the base of the globe clockwise while looking down from the top. Here’s a picture that might help, too.

So one fourth of 25,770 years is 6,443 years. In 6,443 years, the Earth’s tilt will be rotated 90 degrees to the right, or clockwise. Where there was once a solstice, there’s now an equinox, and vice-versa. As you can well imagine, the seasons will happen a whole fourth of the way around Earth’s orbit from where they happened before. Winter will be 3 sidereal months earlier, along with spring, summer, and autumn.

12,885 years from now, Earth’s tilt will be the complete opposite direction from where it is now, and we’ll have seasons on the opposite sides of the sun where we have them now. another 12,885 years and we’re back to having winter where we have it today.

The Tropical Year is shorter because the Earth precesses 1/25770th of the way around each year, which causes the equator to match the ecliptic a little bit before the Earth actually gets around to the same place. 1/25770 of 365 days, 6 hours, 9 minutes, 9 seconds is 20 minutes and 24 seconds, which is the exact difference between the two kinds of year. Neat, huh?

So what does this all mean? First, New Year’s Eve is the beginning of the new Calendar Year, and not the actual tropical year, which happens six hours later. This six hour offset accumulates for four years until we have a leap year, where a day is added in February to make up the difference. So when you partied last night, if you did, it wasn’t even the New Tropical Year yet.

Next, the Tropical Year is shorter than the Sidereal Year, meaning that each New Year, as far as the seasons are concerned, is not quite in the same place that it was a year ago. Most people think that the new year is when we get back to the same place, but its not quite true. To translate is to a distance, the two positions are almost 360,000 kilometers apart.

So now you know that how we measure time is not quite the way it was described to you in school, there are quite a few complications involved. We also base the gregorian calendar year off of the tropical year, correcting it every four years with the leap day. If you want to read more about how the Gregorian Calendar got put together, read Bones, Rocks, and Stars.

Let me explain one last thing. I said that 12:18:18 was the Sidereal New Year, well, that’s measuring it from the end of the last leap year, 2004. You could measure it from any point you like, it doesn’t matter.

How about from your birthdate? Add 6 hours 20 minutes and 24 seconds for every year, and subtract 24 hours for every leap year you’ve lived through and see what you come up with. You might find out that on some years you are celebrating your gregorian calendar birthday a day before, or almost a day after the actual sidereal birthday! At 25 years old, my birthday is now off by 14 hours, 30 minutes, and 45 seconds! And so is my New Year’s this year from my first New Year’s. At 2:30, expect to find me… um, loading firewood into my truck. Eh, it’s gotta be done.

(Note: My calculation may be crude - they add leap minutes to years all the time, probably to make up the difference between the gregorian calendar and the tropical year, so I’m assuming that the two years match up, and we can just add the six hours plus all the 20 minutes and 24 seconds accumulated over my 25 years. But hey, close enough for blogging.)

Who cares about all this? Sure, you care, you read allll the way through this, but how to we get everyone to get interested in these distinctions? Calendar Year, Tropical Year, Sidereal Year, and personal Sidereal New Year? An excuse to party again! (Plus as you get older, your personal sidereal birthday/new year gets later and later - more time for celebration! Party on, fellow humans!

Note: I knew Phil Plait the Bad Astronomer would also put up a post about the new year, and it looks like he did - explaining much of what I just did with more pictures than I could dig up and edit (Not on my home computer) Take a read. He also discussed the Anomalistic Year, which I did not, but I’ve got to point out a problem… he called Midnight the New Year… tsk tsk Phil, that’s a bit of bad astronomy. Of course, every moment is a new year from a year before, but don’t forget those pesky ~6 hours. They add up! Especially if you’re talking about an orbital marker… where 20 minutes and 24 seconds equals 360,000 km. A minor detail.

HAPPY NEW YEAR!