For live links, click to: riverhouses.org/2019-spica 😊
June is the first month of Hercules Term in the River Houses, and as the River Houses Star Calendar will tell you, our great-star-of-the-month for June is Spica, the brightest star in the constellation Virgo. Its formal designation is α Virginis — “alpha of Virgo.” Spica (in Latin) is the ear of grain — the spike — in the left hand of the harvest maiden Proserpina (Persephone in Greek).
If you want to introduce your students to Spica, you can start with some basic astronomy from your homeschool star atlas (riverhouses.org/books):
“Virgo is both a zodiacal and equatorial constellation. Spica, its brightest star, is about 30º southwest of Arcturus in Boötes [next month’s great star] and forms the southernmost point of the Diamond of Virgo. Spica has a magnitude of 1.2 and it’s the 14th of the 20 brightest stars in the sky. The autumnal equinox, where the sun crosses the celestial equator on its journey south, is located close to the star η [eta] Virginis. In the rough square formed by Denebola in Leo [the lion’s tail] and ε [epsilon], γ [gamma] and β [beta] Virginis lies the so-called ‘Field of the Nebulae’ where Herschel discovered 323 nebulae; some of these are visible in a small telescope.“ (Celestron Sky Maps, page 7)
The planisphere on the front of your star atlas will help you locate Spica, and that’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Spica is packed with additional information on everything from astrometrics to cultural history.
Spica is a blue-white giant about seven times the diameter of the sun. It has a small companion that is invisible to the eye (even with a telescope) but that can be detected spectroscopically. The Spica pair is about 250 light years distant — close in astronomical terms — and it’s also quite young: about 12.5 million years old.
Spica is very close to the ecliptic, the line followed by the moon, sun, and planets as they cross the earth’s sky, and so on rare occasions it is briefly covered by them — an event called an occultation. The last occultation of Spica by Venus occurred in the year 1783, and the next will occur in 2197.
Measurements of Spica’s position are believed to have led the ancient astronomer Hipparchus to discover the precession of the equinoxes.
This month, why not take your homeschool students out at dusk and introduce them to this great nearby sun, 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. 🌟