On January 4, 2012, I posted my first BAFact: a short astronomy fact that was brief enough to put on Twitter but informative enough to be interesting. I posted the first one on perihelion – the point in Earth’s orbit when it’s closest to the Sun – and the last one will be a year later.
Because 2012 is a leap year with 366 days, July 5th was the 184th day: the first day of the second half of the year. That means I’m more than halfway done!* Appropriately enough, here’s the July 5 BAFact:
I post the BAFacts on Twitter, Google+ (where I can flesh them out a bit more – and add pictures – since there’s no character limit), and have a complete archive of them on the blog as well. With 180+ already in the bag, reading those should keep you busy for a while!
I generally link them to previous blog posts dealing with the topic in question, but not always. I’ve actually been surprised at how difficult it can be to reduce a topic to 100 or so characters (leaving room for the leading "#BAFact: " and shortened link, plus room for retweets), and how that limits some topics. I have also been surprised to find out I haven’t written about some topics! For example, I was thinking recently of making a BAFact about the nearest known black hole, Cygnus X-1, and discovered I had literally never even mentioned it in a blog post! That’s weird… but by coincidence that got fixed just this last weekend.
So this exercise in brevity has given me new things to write about. I’ll note that there have been arguments over the accuracy of some of the BAFacts, too. Sometimes that’s just due to having to be so brief that the description might be misleading if you don’t click the link; I struggle with those but usually make them as clear as possible, and hope people actually read the post to clarify. And once I really did just make a mistake; as I recently mentioned I didn’t know that recent research had found that zodiacal light is mostly from comet dust and not asteroid collisions, and had to post an immediate correction! But that’s OK; I love learning new things, too.
So as we enter the second half of these, I hope you keep up with them and enjoy them. And if you have a beef with them, find a mistake, have something to add, or know of a good picture or story relating to them, follow it up with a tweet of your own! The whole point here is to have fun and learn things. Which, when it comes to science, are exactly the same.
* Well, kinda. Perihelion is actually on January 2, 2013, roughly a day earlier than usual because we have an extra calendar day this year. The Earth orbits the Sun not caring at all for our calendrical contrivances, so when the time comes I’ll decide whether to post the last BAFact based on the Earth’s orbit our roughly-hewn measurement of it.
Today – July 5, 2012 – at about 04:00 UTC (a few hours ago as I write this) the Earth reached aphelion, the point in its elliptical orbit when it’s farthest from the Sun.
According to the US Naval Observatory, we were 1.016675058 Astronomical Units from the Sun at that time. An AU is the average distance from the Earth to the Sun, and is defined as 149,597,870.7 kilometers (92,955,807.2 miles).
That means that at aphelion the center of the Earth was 152,092,424 km (94,505,851 miles) from the center of the Sun.
Over the next six months we’ll slowly approach the Sun again until we reach perihelion – the closest point in the Earth’s orbit to the Sun – on January 2, 2013, at about 05:00 UTC.
When we’re farther from the Sun it appears a little bit smaller in the sky, but you’d never notice. For one thing, staring at the Sun is a bad idea! For another, the change is so slow day by day that it’s impossible to notice anyway. For a third thing, the total change over the course of six months isn’t very big either. Astronomer (and friend of the blog) Anthony Ayiomamitis took two pictures that show this:
These are from aphelion and perihelion in 2005, but the scale is always about the same every year. As you can see, the change in the Sun’s size isn’t terribly big.
So even though you may not notice it, it’s still neat to think that after the past 183 days or so we’ve been steadily moving farther from the Sun, and now we’re on our way back in. And even neater… the Earth has done this over four and half billion times before. So it has some experience here.
Today is the Fourth of July, Independence Day for us American types.
It also happens to be aphelion*, the point in Earth’s ever-so-slightly elliptical orbit when it’s farthest from the Sun. Perihelion — closest approach — happens in early January, and aphelion six months later. The dates change a bit from year to year because there aren’t an even number of days in a year (that pesky extra 0.24 in the 365.24 days per year messes things up), and there are other minor factors as well.
Today though, aphelion occurs on or about 15:00 UT (11:00 Eastern US time), when the center of the Earth will be about 152,102,715 km (94,512,245 miles) from the center of the Sun — give or take a few hundred meters. If you’re curious, that’s about 1.67% farther from the Sun than on average. That in turn means the Sun appears about 1.67% smaller in diameter than usual, which isn’t noticeable to your eye — and I don’t recommend trying to find out — but is pretty obvious in photographs using telescopes and heavy filtering, like this one from astrophotographer Anthony Ayiomamitis:
Cool, huh? When we’re farther from the Sun we receive a bit less heat, so perhaps those of you suffering from the midwest heat wave can take consolation that it could be worse by a couple of degrees right now.
Later today, coincidentally, I’ll be at a picnic with lots of solar astronomers. What do I say to them? "Hap-helion Fourth of July"? Or, "Enjoy us being at a(1+e) [where a = 1 AU and e = 0.0167] from the Sun today"?
That seems awkward. The thing is, I’m pretty sure a lot of them would get it…
* I pronounce it app-HEEL-eeyun, if you care.
[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn't a math error but a typographical one, and I used the right value when doing my calculations below.]
Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month.
But don’t panic! We do this every year. And that part of it ends today anyway.
The Earth’s orbit around the Sun is not a perfect circle. It’s actually an ellipse, so sometimes we’re closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year — you can get the numbers at the Naval Observatory site — but aphelion (when we’re farthest from the Sun) happens in July, and perihelion (when we’re closest) in January.
And we’re at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we’ll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun.
Of course, since today is when we’re closest to the Sun this year, every day for the next six months after we’ll be a bit farther away. That reaches its peak when we’re at aphelion this year on July 4th, when we’ll be 152,096,155 km (94,507,988 miles) from the Sun.
Not that you’d notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:
This may seem a bit odd if you’re not used to the physics of orbital motion, but you can think of the Earth as moving around the Sun with two velocities: one sideways as it sweeps around its orbit, the other (much smaller) toward and away from the Sun over the course of a year. The two add together to give us our elliptical orbit. The sideways (what astronomers call tangential) velocity is about 30 kilometers (18 miles) per second, which is incredibly fast. But then, we do travel an orbit that’s nearly a billion kilometers in circumference every year!
Does the Sun look a little brighter to you? Maybe that’s because at nine minutes after midnight (UT) tonight, January 2/3, the Earth will be at perihelion, the closest point on its elliptical orbit to the Sun.
At that moment, the Sun’s center will be 147,098,040 kilometers away from the Earth’s center (that’s 91,402,484.5 miles for you Murricans). That is, assuming the distance from the centers of the two bodies is 0.983289667 Astronomical Units, and one AU is 149,597,870.7 kilometers. You can compare that to when we reach aphelion, our most distant point from the Sun, which in 2010 will occur on July 6 at 11:30 UT, when we’ll be 1.016701958 AU or 152,096,448 km (94,508,351.3 miles) from our star.
That change in distance — about 5 million kilometers, or 3 million miles — is only a small fraction of our distance from the Sun, so it doesn’t change the Earth’s temperature very much: a few degrees Celsius, but that’s about it. So, of course, that’s not the reason we have seasons. If it were, then we’d have winter in July in the northern hemisphere! But of course, the international cabal of astronomers covers this fact up.
Still, when you think about it, the Sun is a frakkin’ long way off. Even now, at our closest point, it would take over 20 years to fly to the Sun in an airplane at 800 kph (500 mph)! And the TSA would make you sit silently with nothing in your lap for the last 3 years of the journey, too.
But my point is (in case you were wondering if I had one) that the Sun is hot, and there’s a lot of it. I’m glad it’s so far away, even when it’s at its closest.