And so a new year begins. Interestingly enough, there really is no significance as to this date starting the new year, other than historical references. Astronomically, this date doesn’t signify anything; the winter solstice is slightly more significant astronomically, since it signifies when the days start to lengthen again rather than shorten. This is all tied into the Earth’s orbit around the sun. How we measure that orbit is quite a bit more complex than what our calender surmises. Bad Astronomer, one of my favorite blogs, explains it exceptionally well in his post about the tropical year: How Astronomers define a year
A “day” is how long it takes the Earth to rotate once, but we’re back to that measurement problem again. But hey, we used the stars once, let’s do it again! You stand on the Earth and define a day as the time it takes for a star to go from directly overhead to directly overhead again: a sidereal day. That takes 23 hours 56 minutes 4 seconds = 86,164 seconds. But wait a second (a sidereal second?)—shouldn’t that be exactly equal to 24 hours? What happened to those 3 minutes and 56 seconds?
I was afraid you’d ask that—but this turns out to be important.
It’s because the 24-hour day is based on the motion of the Sun in the sky, and not the stars. During the course of that almost-but-not-quite 24 hours, the Earth was busily orbiting the Sun, so it moved a little bit of the way around its orbit (about a degree). If you measure the time it takes the Sun to go around the sky once—a solar day—that takes 24 hours, or 86,400 seconds. It’s longer than a sidereal day because the Earth has moved a bit around the Sun during that day, and it takes a few extra minutes for the Earth to spin a little bit more to “catch up” to the Sun’s position in the sky.
A diagram from Nick Strobel’s fine site Astronomy Notes (shown here; click to embiggen) helps explain this. See how the Earth has to spin a little bit longer to get the Sun in the same part of the sky? That extra 3 minutes and 56 seconds is the difference between a solar and sidereal day.
OK, so we have a year of 31,558,149 seconds. If we divide that by 86,164 seconds/day we get 366.256 days per year.
Wait, that doesn’t sound right. You’ve always read it’s 365.25 days per year, right? But that first number, 366.256, is a year in sidereal days. In solar days, you divide the seconds in a year by 86,400 to get 365.256 days.
Phew! That number sounds right. But really, both numbers are right. It just depends on what unit you use. It’s like saying something is 1 inch long, and it’s also 2.54 centimeters long. Both are correct.
Having said all that, I have to admit that the 365.25 number this is not really correct. It’s a cheat. That’s really using a mean or average solar day. The Sun is not a point source, it’s a disk, so you have to measure a solar day using the center of the Sun, correcting for the differences in Earth’s motion as it orbits the Sun (because it’s not really a circle, it’s an ellipse) and and and. In the end, the solar day is really just an average version of the day, because the actual length of the day changes every, um, day.
Quote from Phil Plait’s blog: Bad Astronomy
As you can see, determining the length of year can be quite complex. The article goes on to explain adapting a calender to the above, how the Earth’s orbit around the sun plays into all of this, and other fascinating astronomical tidbits about our solar system and how we measure time. For the most part, picking January 1st is slightly arbitrary and mostly dependent on history, but as for the measurement of the year and how many days are in our year, that’s astronomically based. It’s also not an integral number, and thus to account for that, we have leap years to essentially ‘reset’ our calenders in order to avoid having our calenders not match up with the seasons. That’s all explained extremely well in the link above, so I don’t really feel the need to discuss it too heavily here. For me, a lover of astronomy, I find this all fascinating.
And so it’s a new year, and another new start. This past year of 2013 has been extremely harsh and unforgiving for me, and held only a handful of good moments, but it is my hope that this upcoming year will be much kinder and more favorable.
Also, I hope to write more short stories this year. It’s a goal, so that I can attempt to get more of them published and to beef up my writing resume. Another hope is to finish the novella I’m working on and to edit and revise my Snow White novella as well.