Stardust and eternity – 3.3.2

Circumpolar and seasonal constellations

Among the 6000 stars visible to the naked eye at ideal locations, we often recognize particular groups of stars situated in the same region of the sky, forming particular patterns or “asterisms” – commonly known as constellations. Nowadays, the astronomical community acknowledges 88 constellations, the majority of which are visible from the northern hemisphere of our planet. Although stars belonging to the same constellation may be very far from one another, these stellar patterns are popularly known for historical reasons and frequently bear mythological names, based on classical Greek legends, such as Andromeda, Gemini, Orion, and so on. Constellations have always been useful tools for sky orientation. For instance, the star Polaris, or “Little Dipper” – a moderately bright star at the tip of the handle of the Ursa Minor constellation – has been used by navigators since ancient times to determine the direction of north. Indeed, the Earth’s axis points almost exactly to Polaris; all other stars, due to Earth’s rotation, appear to circle counterclockwise around this “fixed” star at a speed of about 15 degrees per hour, completing one complete revolution (or 360 degrees) in one day.

Since the altitude of Polaris above the horizon is equal to the observer’s latitude, some of the stars that lie close to it – and thus their constellations – never descend below the horizon as they make their apparent paths around it. These “circumpolar constellations” are thus visible all year above the horizon. For example, the well-known Big and Little Dipper constellations (Ursa Major and Ursa Minor), or the W-shaped Cassiopeia are always visible in any season and every night at European latitudes, whereas at the North Pole all stars are necessarily circumpolar. Conversely, there are no circumpolar constellations at the Earth’s equator. Moreover, constellations observed by stargazers at southern latitudesm, near the South Pole, never rise above the horizon in the northern hemisphere, so that they are always invisible to us.

Differently from circumpolar constellations, the so-called “seasonal constellations” are visible at night only in particular seasons. Indeed, since our planet changes position as it orbits around the Sun throughout the year, we continuously see the stars of our galaxy from a different perspective – according to the patterns of stars on the night side of the Earth, opposite to the Sun. For example, given that the Sun’s apparent path in the northern hemisphere transits through three of the twelve “zodiacal constellations” of western astrology during winter – Sagittarius, Capricorn and Aquarius – these constellations are not visible in the winter night sky. They become instead visible during summer, when our field of vision has shifted 180 degrees.

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Gemini, Orion, Taurus constellation and Pleiads on Mount Rosa – Photo by E.Balboni – Cosmoedintorni.org

Little and Big Dipper over lake Vallette – Photo by E.Balboni – Cosmoedintorni.org

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Images

Gemini, Orion and Taurus over a cascade (E.Balboni, cosmoedintorni.org)