In the constellation of Vulpecula, the fox – located high in the sky this time of year for northern hemisphere observers – is a fun little asterism: a collection of stars formally known as Brocchi’s Cluster, or Collinder 399. Greek astrophotographer Anthony Ayiomamitis took a grand picture of it just a few days ago that I have to share with you:
[Click to enhaberdasherate.]
It’s very pretty, isn’t it? The asterism itself is composed of the ten or so brightest stars you see; the rest are background stars. It’s most likely not a true cluster; that is, the stars may be at different distances and not physically associated with one another.
Still, this so happens to be one of my all-time favorite objects in the sky. Why? Because of the shape of the cluster: it really looks like a coathanger! If you don’t see it, I drew lines between the stars in question in the image inset here.
Now you see it, right? Astronomy Picture of the Day featured a different shot of the Coathanger in 2008, too, which is worth a look.
This group of stars is pretty big – three times the width of the full Moon on the sky – and bright, making it really easy to spot with binoculars. In fact, when I was younger I stumbled on it myself by accident while scanning that part of the sky with my telescope. It was obviously shaped like a coathanger, and I was delighted to find out that’s what everyone called it.
In fact, now that I think about it, it’ll be very well placed for observing in September, making it a great target for folks coming to Science Getaways. That would be fun. But if you have clear skies this time of year, it’s easy to find, about a third of the way from the bright star Altair to Vega, in the Summer Triangle. Give it a shot!
Image credit: Anthony Ayiomamitis, used with permission.
[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!
When Pluto was discovered 80 years ago, it happened to be moving through Gemini, a part of the sky that had a lot of stars. Clyde Tombaugh did an amazing job finding it, since it was almost lost among those stars.
I wonder if he could’ve found it had he been looking earlier this year? "Amateur" astronomer Anthony Ayiomamitis sent me this image he took of Pluto as was in Sagittarius, the most densely-packed area of the sky!
[Click to undwarfplanetate.]
Hard to spot, isn’t it? Pluto is unresolved in the picture, so it looks like just another star. And there are a lot of stars here; this region of sky is actually a cluster called Messier 24 (or just M24, and it’s pronounced "MEZ-ee-ay", since Charles Messier was French); the two dark splotches are thick dust clouds called Barnard 92 and 93. Finding Pluto in this ain’t easy.
I’ve been posting a lot of extreme close-ups of the Moon, but sometimes you can learn something by taking a step back.
For example, I imagine if I went out in the street and asked people what shape the Moon’s orbit was, they’d say it was a circle (or, given recent poll results, they’d say it was Muslim). In fact, however, the Moon’s orbit is decidedly elliptical. When it’s closest to Earth — the point called perigee — it’s roughly 360,000 kilometers (223,000 miles) away*, and when it’s at its farthest point — apogee — it’s at a distance of about 405,000 km (251,000 miles).
That’s a difference of about 10% — not enough to tell by eye, but certainly enough to see in a picture… like this one, by the Greek amateur astronomer Anthony Ayiomamitis:
[Click to emperigeenate.]
Amazing, isn’t it? The Moon is noticeably different! He took those images at full Moon, but seven months apart, when the Moon was at perigee (last January) and apogee (just a few days ago as I write this). It’s part of a project he does every year, and it’s pretty cool. He was able to get these images within a few moments of the exact times of apogee and perigee.
You might wonder how the Moon can be at apogee when it’s full one time, and perigee at another time it’s full. Read More
"I am constant as the northern star,
Of whose true-fix’d and resting quality
There is no fellow in the firmament."
Julius Caesar (III, i, 60 – 62)
Shakespeare was a decent writer, but an astronomer he wasn’t. The North Star isn’t fix’d, because the Earth’s axis wobbles slowly like a top. You wouldn’t see this by eye, since the circuit takes 26,000 years to complete, but astronomers deal with it all the time.
But Shakespeare did get something right in that passage: the stars themselves do move. It’s slow, but it’s there. It’s caused by their orbital motion as they circle the center of the Milky Way. Their velocity can be hundreds of kilometers per second, but that apparent motion is dwarfed to a near standstill by their forbidding distance. Of course, that means that closer stars will appear to move faster than ones farther away, just like trees by the side of the road whiz by as you drive, but distant mountains slide along in a much more stately manner.
It takes decades, sometimes, to see that stellar movement at all — astronomers call it proper motion — but it’s not impossible. Greek amateur astronomer Anthony Ayiomamitis (who has been featured on this blog before here and here) knew that very well, and he was able to prove it. Behold, the unfix’d heavens!
These two pictures show the same region of sky, separated in time by six decades. The top, taken in 1950, is from the famous Palomar Sky Survey, a tool still used by astronomers to guide their observations. The marked star is Barnard’s Star, a dinky, dim red bulb a mere 6 light years away — which makes it one of the closest of all the stars in the galaxy.
Yesterday, I wrote about the comet 2009 R1 McNaught which is currently in the extreme northern sky in the early morning. By coincidence, just hours after posting it, I got an email from the amateur astronomer Anthony Ayiomamitis (the same guy who took the very cool picture of the ISS and Jupiter in the daytime), who sent me this picture of the comet he took in Greece at just around the same time that post went live:
Wow, very pretty! The solid part of the comet, called the nucleus, is far smaller than a single pixel in this image, since the comet was more than 175 million km (110 million miles) away when he took this shot. The nucleus of even a huge comet is only a few dozen km across, so at that great distance is just a tiny dot. Anthony has details on his observations on his McNaught page.
The comet looks huge — and the fuzzy part can be bigger than planets! — because what you’re seeing is gas expanding away from the nucleus. Far from the Sun that gas is frozen, and the comet is solid. But heat it up, and that ice turns into a gas, creating the comet’s coma (Latin for hair). In that gas methane, water, ammonia, and lots of other things, many of which are pretty nasty.
But why is it green?
Ah, that’s a good question (I’m glad I asked it!) and takes just a little bit of background.
The amazing pictures of the space station taken by ground-based amateur astronomers keeps on coming. On May 29th, Anthony Ayiomamitis used a 16 cm (6″) telescope to capture a phenomenal image of the International Space Station passing Jupiter… in broad daylight!
Wow! Note the color of the sky; it was about 9:00 a.m. local time when he took this shot, with the Sun well above the horizon. This is actually two images added together; the first shows the ISS to the lower right, and in the second shot it had moved to the upper left. Jupiter shows its disk near the center of the frame, it being easily bright enough to be seen using a telescope in daylight.
What an incredible picture! But it gets cooler…