Year
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Another year, another repost: The article below is an updated version of one I try to post every year at this time — either because the topic is so much fun, or I’m lazy. Take your pick. But I love this kind of stuff; it’s fun to research and to play with the numbers. If you like it too, read on. If you don’t, read it anyway, because you might find out you do, and isn’t that one reason we celebrate the new year? To try out and experience new things, or old things anew? You might also want to read about why we have leap years and even leap seconds. Science! I love this stuff.
Yay! It’s a new year!
But what does that mean, exactly?
The year, of course, is the time it takes for the Earth to go around the Sun, right? Well, not exactly. It depends on what you mean by “year” and how you measure it. This takes a wee bit of explaining, so if you’re done kicking 2016 to the curb and trying your best to hope for 2017, sit back and let me tell you why we have a new year at all.
Round and Round She Goes
Let’s take a look at the Earth from a distance. From our imaginary point in space, we look down and see the Earth and the Sun. The Earth is moving, orbiting the Sun. Of course it is, you think to yourself (unless you’re a Geocentrist, in which case this stuff still all works, just the other way around). But how do you measure that? For something to be moving, it has to be moving relative to something else. What can we use as a yardstick against which to measure the Earth’s motion?
Well, we might notice as we float in space that we are surrounded by billions of pretty stars. We can use them! So we mark the position of the Earth and Sun using the stars as benchmarks, and then watch and wait. Some time later, the Earth has moved in a big circle (OK, ellipse, but they’re pretty close in this case) and is back to where it started in reference to those stars. That’s called a “sidereal year” (sidus is the Latin word for star). How long did that take?
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 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.
The Sun Rose by Any Other Name
Confused yet? Yeah, me too. It’s hard to keep all this straight. But back to the year: That year we measured was a sidereal year. It turns out that’s not the only way to measure a year.
You could, for example, measure it from the exact moment of the March equinox (also northernhemispherictically sometimes called the vernal equinox) —a specific time of the year when the Sun crosses directly over the Earth’s equator in March— in one year to that same equinoctal moment in the next. That’s called a tropical year (which is 31,556,941 seconds long). But why the heck would you want to use that? Ah, because of an interesting problem! Here’s a hint:
The problem, though, is that the calendar year doesn’t line up with them well. The date of perihelion changes year to year due to several factors (including, of all things, the Moon, and the fact that we have to add a leap day roughly every four years). In 2013 perihelion was on Jan. 2, but in 2017 it’s on Jan. 4. Same thing with the equinox: It can range from March 20 to March 21. That makes using orbital markers a tough standard.
Various countries used different dates for the beginning of the year. Some had already used Jan. 1 by the time the Gregorian (tropical) calendar was first decreed in 1582, but it took time for others to move to that date. England didn’t until 1752 when it passed the Calendar Act. Not surprisingly, there was a lot of religious influence on when to start the new year; for a long time a lot of countries used March 25 as the start of the new year, calling it Lady Day, based on the assumed date when the archangel Gabriel told Mary she would be the mother of God. Given that a lot of ancient Christian holidays are actually based on older, Pagan holiday dates, and the fact that this was on March 25—very close to the equinox—makes this date at the very least suspicious.
Still, in the end, the date to start the new year is an arbitrary choice, and Jan. 1 is as good a day as any. And as a happy side effect it does help establish the Knuckle Rule.
Resolving the New Year
So there you go. As usual, astronomers have taken a simple concept like “years” and turned it into a horrifying nightmare of nerdery and math. But really, it’s not like we made all this stuff up. The fault literally lies in the stars and not ourselves.
Now if you’re still curious about all this even after reading my lengthy oratory, and you want to know more about some of these less well-known years, then check out Wikipedia. It has lots of info, but curiously I found it rather incomplete. Every year (take your pick which kind) I say to myself I’ll submit an updated article to Wikipedia listing all the different years and the number of seconds and days of each kind in them.
Then every year I forget. But if you want to give it a shot, feel free. It would come in handy when I update this article every 365.26 days or so.
Incidentally, after all this talk of durations and lengths, you might be curious to know just when the Earth reaches perihelion, or when the exact moment of the vernal equinox occurs. If you do, check out the U.S. Naval Observatory website. It has tons of gory details about this stuff.
And, finally (for real this time) I have to add one more bit of geekiness. While originally researching all this, I learned a new word! It’s nychthemeron, which is the complete cycle of day and night. You and I, in general, would call this a “day.” Personally, if someone dropped that word into casual conversation, I’d challenge them to a duel with orreries at dawn.
Hmmmm, is there anything else to say here? (Counting on fingers.) Years, days, seconds, yeah, got those. (Mumbling.) Nychthemeron, yeah, Gregorian, tropical, precession, anomalistic … oh wait! I know something I forgot to say:
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