I have a question for you. Do you know the name of the brightest nighttime star we see most often? Yes, this is a bit of a trick question. No, it's not Polaris the North Star, nor is it Sirius, the brightest star in the nighttime sky. It's not even my favorite star, Betelgeuse, the armpit of the constellation Orion the Hunter.
It turns out the brightest star we see most often in the night sky is Capella, known affectionately as the "goat star." During these mid-spring evenings when it's truly dark enough, around 10 p.m., you'll find Capella hanging out in the low western skies. Normally this time of year, Capella would be the brightest star-like object in that part of the sky, but this spring, Capella has to take a back seat to the planet Jupiter.
This is a good time to remind you how to tell the difference between a planet and a star when you gaze into the night sky: Stars twinkle and planets don't. That's because stars are so much farther away, and the single light wave they send our way gets bounced around by the winds in our atmosphere.
Since planets are so much closer, they send multiple light rays our way, and they compensate each other when they come through Earth's buoyant atmosphere. I have to tell you though, that even planets can twinkle a little bit when they're close to the horizon, where Earth's atmosphere is a lot thicker from our perspective.
Near Polaris
Capella is the fourth brightest star we see through the course of the year, but it's the brightest star we see most often in the northern hemisphere. That's because it's the nearest, brightest star in the sky to Polaris, the North Star.
Polaris marks the position of the north celestial pole. Every celestial object we see in the sky day or night, whether it's the sun, the moon, planets or stars, all rotate once around the North Star Polaris once every 24 hours. Polaris is the "lynchpin" (sorry) of the sky because it shines directly above the Earth's terrestrial North Pole.
If we lived at the North Pole, the North Star would be directly overhead and everything in the celestial dome would pivot around the overhead North Star every 24 hours. In Pennsylvania, we live about halfway between the North Pole and the Earth's equator, so Polaris in our sky is permanently fixed directly about halfway between the northern horizon and the overhead zenith.
Stars close to Polaris in the sky, such as those that make up the Big and Little Dippers as well as the W-shaped constellation Cassiopeia, are so close to the north celestial pole that they are always above the horizon in a tight circle around the North Star. They are called circumpolar stars, and we see them night after night.
Capella the goat star is not quite close enough to Polaris to be considered a circumpolar star, but it's close. Because of its northwardly position, Capella can be seen in our evening skies from about late August to mid-June. Throughout the year, it never goes an entire night without making at least a brief appearance.
Capella is called the goat star because it's the brightest star in the constellation Auriga the Charioteer. The constellation Auriga basically resembles a lopsided pentagon that, according to Greek and Roman mythology, is supposed to be a retired chariot driver turned goat farmer with a mama goat on his shoulder and baby goats in the crook of his elbow.
How you get all of that out of a lopsided pentagon is beyond me. There must have been quite a party when the Greeks and Romans dreamed up that constellation. I wish I could have been there.
Capella is supposed to mark the position where the mama goat is sitting on the chariot driver's shoulder, and that's why it's known as the "goat star." For extra credit, when you look for the pentagon of Auriga, which is pretty easy to spot in the sky, look for three dim stars shining just below Capella. Those are the baby goats or kids in the crook of the drivers elbow. The closest kid star to Capella is a star called Almaaz. It looks like a puny star, but don't let looks deceive you. It just looks small and dim because it's so far away at about 12,000 trillion miles from Earth. It's at least a hundred times the diameter of our sun, and more than 40,000 times as luminous. That's one big, bright kid.
Celestial hugging
The waning crescent moon will be up close and personal with the bright planet Venus in the early morning low eastern next Saturday morning. The moon will be parked just to the left of the very bright planet.
(Lynch is an amateur astronomer and author of the book, "Stars, a Month by Month Tour of the Constellations." Contact him at mikewlynch@comcast.net.)