Every August, stargazers across the world are treated to the Perseid meteor shower, often considered the best meteor shower of the year. This year, the Perseids reached their peak just before dawn on Aug. 12, with good viewing conditions expected to continue for the next few days. Many Dartmouth students enjoyed the shower, and some attended a lecture about meteors by earth sciences professor Brenhin Keller at Moosilauke Lodge on Sunday. The Dartmouth sat down with astronomy professor emeritus John Thorstensen to learn more about the Perseids and other wonders of the night sky.
What are meteor showers and why do they happen?
JT: Meteor showers arise when comets — which come from the far reaches of the solar system — get perturbed and dive in toward the sun. A comet is best thought of as a dirty snowball. It’s a combination of ice and particulate matter left over from very early in the formation of the solar system. When comets get close to the sun, they start to sublimate. The pressure is so low that they don’t actually melt into water — they just turn into gas. These particles that are embedded in the comet — think of a dirty snowbank melting — start to stream off into space. When the Earth’s orbit carries it through this stream of particles, we see them as they hit the atmosphere.
What do we know about the comet that the Perseids come from?
JT: This particular comet is known as Swift-Tuttle. It was discovered back in the 19th century and has been slowly breaking up over time. Astronomers realized that the orbit of Swift-Tuttle is very similar to the orbits of all these Perseid meteors. That’s where the Perseids came from — they are particulates that basically boiled off Swift-Tuttle and are retaining the same orbit. In space, there is no air and no friction, so these particles can orbit for a long time.
What makes the Perseids so visible in the night sky?
JT: When we pass through this cloud of debris, the relative speed between the earth and the cloud of debris is very high. It’s more than 50 kilometers per second. When the debris strikes the atmosphere, it is carrying huge amounts of energy –– vastly more than, say, a rifle bullet of the same mass. These particles immediately vaporize, and as they do, they leave a trail of glowing gas because they get so hot. The particles break up very quickly in the atmosphere, and you can see the particle moving along and the glowing gas that it leaves.
Why are the showers called the Perseids?
JT: They’re called the Perseids because the so-called radiant — the spot in the sky in which it looks like all the meteors are coming from — is near the constellation Perseus. Originally, the radiant was right in the constellation, but it’s moved slightly over time so it’s now closer to Cassiopeia, but that’s very close by.
Are there any other meteor showers that have been particularly notable in history?
JT: There have been meteor showers in history where a young comet had just boiled off, and the Earth happened to go right through the densest part of the particles around it. In 1833, the Leonids — a meteor shower from a totally different comet — just happened to hit Earth’s orbit so spectacularly that people said they thought the world was ending. There were meteors crossing the sky all the time. It was also before electric lighting, and people came out of their houses and went, “Oh my God, what the hell is happening?”
Do you have any tips for viewing the Perseids?
JT: Go out in a nice, dark place at the right time, and basically stare at the sky. There’s no instrument that you can use — the best thing is to use your eyes. If you just lie there, you can typically see a meteor every few minutes. One thing to be aware of is that the spot in the sky from where it looks like the Perseid streaks are all coming doesn’t rise in the sky until about midnight. So, the Perseids tend to be best in the early morning hours. The other thing to be aware of right now is that the moon is just past half, so the meteor shower is going to move into the morning sky. In the earlier part of the moon cycle, the meteor shower is more visible in the evening sky, but then it moves into the morning sky.
What else should we be looking for in the night sky right now?
JT: Another thing to see in the early morning hours is that Jupiter and Mars are close together. Jupiter is the very bright star-like object in the eastern sky in the morning, and Mars is a little red dot really close to it. They’re not close together in space — they’re in very different orbits — but our perspective is such that they appear in almost the same line of sight.
What kinds of observational astronomy does the astronomy department do on campus?
JT: We’re thinking of restarting public nights pretty soon, but we haven’t done it yet. The telescope in Shattuck Observatory on campus is not operational right now. It’s a historic thing, which we’re looking at bringing back into operation. We do most of our research in two different observatories way off campus. One of them is in Arizona and the other one is in South Africa.
When did you first become interested in astronomy?
JT: I almost can’t remember not being fascinated by the night sky. One thing that has always impressed me is how essentially everything we think about is on this tiny little dot of a planet, and there is this enormous world out there that most people know nothing about. I remember, as a kid, just being amazed at interstellar distances and things like that. The world has many connections and so much fascinating stuff going on.
This article has been edited for clarity and length.
