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March is the first month of Leo Term in the River Houses, and as our monthly star calendar will tell you, March’s Great Star is Sirius, the brightest star in the constellation Canis Major, the Big Dog. Its formal designation is α Canis Majoris — “alpha of Canis Major.” Sirius is brilliant in the southern sky at sunset now each evening, trailing along behind Orion and passing to the west as the night goes on.
If you want to introduce your students to Sirius and Canis Major, you can start with some basic astronomy and astronomical mythology from your backyard star guide (riverhouses.org/books):
“Canis Major is located near the Milky Way, just to the east of Orion. The constellation is most easily spotted by locating its alpha star, Sirius — the brightest star in the night sky. Only 8.6 light-years away, it shines a brilliant magnitude of –1.5 and has a close companion white dwarf, though it’s difficult to spot even with a 10-inch (250 mm) telescope. Sky-watchers can identify Sirius by tracing a line through Orion’s belt and continuing southeast, where the bright star marks the northern edge of Canis Major.
“Canis Major is considered to be the larger of Orion’s two hunting dogs. The phrase “dog days of summer” takes its origin from Sirius, known as the Dog Star. In late summer in the Northern Hemisphere, the star rises around the same time as the sun, leading to the belief that its heat and brightness help to bring extra warmth to the north.“ (Backyard Guide to the Night Sky, page 269)
That’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Sirius is packed with additional information on everything from astrometry to cultural history.
Sirius is indeed a double star system, but the smaller star of the pair is only visible in large telescopes. The name Sirius means burning or scorching in Greek (Σείριος), either from its brightness or from the timing of its dawn rising along with the sun in mid-summer. (Today, we tend to make seasonal associations with the stars and constellations according to their evening visibility, but in ancient times, it was the dates on which particular stars rose just ahead of the sun at dawn — the dates of heliacal rising — that were commonly used for calendrical purposes.)
Sirius is roughly twice the diameter of our sun, and it is much younger — less than 250 million years old, in contrast to the sun’s 4.6 billion year age. Sirius was one of the first stars whose “proper motion” against the distant background was confirmed. The astronomer Edmond Halley (1656–1742), by comparing contemporary measurements of the position of Sirius with those of the ancient astronomer Claudius Ptolemy (ca. 100–170), determined that Sirius had moved about 30 arc-minutes across the sky since Ptolemy’s day — a positional change in 1500 years equivalent to the diameter of the moon. The “fixed stars” were not so fixed after all.
Sometime this month, take your homeschool students out at dusk and introduce them to this brilliant nearby sun, and teach them its name, and so give them a new friend for life.
What stellar observations will you be making in your homeschool this Leo Term? 😊
❡ 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 mark the top of Orion’s sword, so even though it’s not very bright it’s still notable and easy to locate on a dark night. ✨
❡ Star bright: The brightness of a star as we see it in our night sky is its magnitude — or more properly, its apparent magnitude. The scale of star magnitudes was developed long before modern measuring instruments were invented, so it can be a little bit confusing for beginners. Originally, the brightest stars in the sky were called “first magnitude” and the less-bright stars “second magnitude,” “third magnitude,” and so on, down to the dimmest stars visible to the naked eye, which were called “sixth magnitude.” In the nineteenth century the star Vega (our August star) was chosen as the standard brightness reference and its value on the magnitude scale was defined to be zero (0.0). Five steps in magnitude (from 0.0 to 5.0 or from 1.0 to 6.0) was defined to be a change in brightness of 100 times: a star 100 times dimmer than Vega (0.0) was defined to be a magnitude 5.0 star. Vega is not quite the brightest star is the sky, however, so the scale also had to be extended into negative numbers: Sirius (our March star), for example, is magnitude –1.5, about three times brighter than Vega (at 0.0). The planet Venus at its brightest is about magnitude –4.2; the full moon is about magnitude –12.9; the sun is magnitude –26.7. By contrast, the dimmest stars visible to the naked eye in a populated, light-polluted area are about magnitude 3.0; the dimmest stars visible under very dark conditions are about magnitude 6.5. The Hubble Space Telescope in orbit around the earth has photographed distant stars and galaxies below magnitude 30, the dimmest celestial objects humans have seen so far. 🔭
❡ And all dishevelled wandering stars: How far away are the stars? Do they all occupy a single celestial “dome” that rotates through the heavens (as some ancient and medieval astronomers believed), or are they scattered through space at different individual distances? Astronomers had long suspected that the fixed stars existed at different distances from us, but early attempts to measure those distances failed. It was not until the early 1800s that instruments and measuring techniques became precise enough to allow the first stellar distances to be calculated using the technique of parallax. Parallax is the displacement in the apparent position of an object with respect to the background when an observer moves from side to side. It’s an ordinary phenomenon you experience every day — it’s how we judge distances as we move through the landscape. Stellar parallaxes are extremely small — fractions of an arc-second (one 3600th of a degree) — and they are calculated by measuring a star’s position against the background at opposite sides of the earth’s orbit, six months apart. (That’s the astronomical equivalent of taking one step to the side.) Vega, our August star, was one of the first stars to have its parallax measured; modern estimates put it at about 0.13 arc-seconds. Apply some trigonometry, and that yields a distance of about 25 light-years. 🔭
❡ Watchers of the skies: Teaching your students to recognize the constellations is one of the simplest and most enduring gifts you can give them. We recommend the handy Backyard Guide to the Night Sky (riverhouses.org/books) as a general family reference — it will help you identify all the northern hemisphere constellations and will point out many highlights, including the names and characteristics of the brightest stars. Your recommended 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 regular Homeschool Astronomy posts featuring twelve of the most notable 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 — and make each one of them a homeschool friend for life. 🌟