For live links, click to: riverhouses.org/2019-polaris 😊
May is the last month of Leo Term in the River Houses, and as the monthly River Houses Star Calendar will tell you, our great-star-of-the-month for May is Polaris, the North Star or Pole Star in the constellation Ursa Minor, the Little Bear. Its formal designation is α Ursae Minoris — “alpha of Ursa Minor.”
Polaris is not an especially bright star — only magnitude 2 or so — and there’s nothing intrinsically special about it. Nevertheless, it’s one of the most important stars in the sky from our terrestrial human point of view because the earth’s orientation in space happens to put Polaris directly above our north pole. That means that as the earth rotates each day on its axis, all the other stars turn through the sky, but Polaris keeps its place: it’s a fixed point in the otherwise turning sky.
Which way is north? Look to Polaris. Want to know your latitude in the northern hemisphere? Your latitude corresponds to the angular elevation of Polaris above a level horizon. If you were at the equator, Polaris would be exactly at the horizon all night, and if you then walked your way to the north pole, Polaris would be at a higher and higher point each night until you reached the pole, when it would be 90º up from the horizon — straight overhead at the zenith.
Polaris is the alpha star — the brightest star — in the small constellation Ursa Minor, the Little Bear or Little Dipper. It’s the star at the end of the Little Dipper’s handle, and you can find it by locating the Big Dipper (Ursa Major), which is generally more conspicuous, and following the two “pointer” stars of the Big Dipper across the sky to Polaris.
The planisphere on the front of your star atlas (riverhouses.org/books) will help you locate Polaris, and that’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Polaris is packed with additional information on everything from astrometrics to cultural history.
The Polaris we see as a single star is in fact a triple star system. The primary star of the system — the source of nearly all the light that comes to us — is a yellow supergiant more than 30 times the diameter of the sun. It has a tiny close companion that can only be detected in powerful telescopes, and a more distant companion that can be fairly easily resolved by amateur observers. The Polaris system is about 400 light years distant — the light we see when we look at Polaris was actually emitted by the star about 400 years ago.
“I am as constant as the northern star,” says Shakespeare’s Julius Caesar. This month, take your homeschool students out at dusk and introduce them to this great fixed star, and teach them its name, and so give them a new friend for life.
What stellar observations have you made in your homeschool lately? 😊
❡ Alpha and beta and gamma, oh my: Most of the principal stars within each constellation have both old vernacular names — Vega, Sirius, Arcturus, and so on — as well as more formal scientific designations. The German astronomer Johann Bayer (1572–1625) devised the formal system of star designations that is still in common use today. In Bayer’s system, the stars in each constellation, from brightest to dimmest, are assigned a lowercase letter of the Greek alphabet: α (alpha, brightest), β (beta, second brightest), γ (gamma, third brightest), δ (delta, fourth brightest), and so on. This letter designation is combined with the Latin name of the constellation in its possessive (genitive) form: Lyra becomes Lyrae (“of Lyra”), Canis Major becomes Canis Majoris (“of Canis Major”), and so on. The brightest star in the constellation Lyra (the star Vega) thus becomes α Lyrae (“alpha of Lyra”), the brightest star in Canis Major (the star Sirius) becomes α Canis Majoris (“alpha of Canis Major”), and so on, through all 24 Greek letters and all 88 constellations. How bright would you expect, say, the σ (sigma) star of Orion to be? Not very bright — it’s far down the alphabet — but σ Orionis happens to be the top star of Orion’s sword, so even though it’s not very bright it’s still notable and easy to locate on a dark night.
❡ Watchers of the skies: Teaching your students to recognize the constellations is one of the simplest and most enduring gifts you can give them. The planisphere on the front of your River Houses star atlas (riverhouses.org/books) will let you dial up the northern hemisphere sky for any night of the year, and the descriptions and maps of each constellation will point out the highlights, including the names and characteristics of the brightest stars. Your general world atlas also has beautiful maps of the whole northern and southern hemisphere night skies on plates 121–122. Why not find a dark-sky spot near you this month and spend some quality homeschool time beneath the starry vault. 🌠 🔭 🌌 🌟
❡ First star I see tonight: This is one of our monthly “Great Star” posts featuring twelve of the brightest stars of the northern hemisphere night sky. Download and print your own copy of our River Houses Star Calendar (riverhouses.org/calendars) and follow along with us as we visit a different Great Star each month. 🌟