Stardust and eternity – 3.3.1

Orienting yourself in the night sky

The night sky has always been an impressive and inspiring sight for mankind, especially before the technological era, when light pollution was not as dominant as today. Out of necessity, ancient civilizations were more attentive to the cycles of nature out and therefore developed their own systems to identify and map the stars in the sky. They ideally connected stars with imaginary lines to form patterns or “constellations”, which were often given mythological names surviving to this day. Constellations helped them understand the night sky. For this purpose, ancient civilisations such as the Babylonians, the Egyptians, or the pre-Columbian Maya (to name a few) also built unique, elaborate and imposing stone structures that were used to study astronomical positions and events. The reasons for this were mainly practical, as the repetition of the celestial objects observed in the sky each year made it possible both to orient oneself on Earth (or to navigate seas) and to plan agriculture, due to the Earth’s revolution around the Sun.

In this regard, a moderately bright star known as Polaris – the tip of the handle of Ursa Minor, or Little Dipper – was probably one of the most important celestial objects. Indeed, as this star remains fixed pointing north at a latitude almost equal to that of the observer, it has historically been used by navigators to determine the direction in which to travel at night. All other stars move in circular paths around it.

Even today, constellations serve as a kind of map of the night sky, especially for astronomical purposes. The International Astronomical Union has indeed divided the sky into 88 official constellations with definite boundaries, so that each star within these boundaries has the constellation’s name preceded by a Greek letter, according to its brightness. For example, the red supergiant Betelgeuse, the brightest star in the constellation Orion, is designed as “α Orionis”, while the second brightest, the blue supergiant Rigel at the left foot of Orion, is named “β Orionis”.

While constellations such as Orion or Ursa Major are easily and neatly identifiable at night (even with moderate light pollution), many others consist mostly of fainter star and are therefore harder to identify under normal stargazing conditions. A useful example is the aforementioned constellation Ursa Minor, whose seven stars – composing the “dipper” – become progressively fainter, starting with Polaris (α Ursae Minoris), and Kochab (β Ursae Minoris); they both have an apparent magnitude of about 2.0. However, other stars – from brightest to dimmest – have apparent magnitudes of 3.1, 4.2, 4.3, 4.4, and 5.0 and are therefore much fainter. Recall that the apparent magnitude is a scale of a star’s brightness measured on Earth on a logarithmic scale: higher numbers correspond to fainter brightnesses (so that a star of magnitude 2.0 is 25 times brighter than a +5.5 magnitude star!). As a principle, since the theoretical limiting magnitude for naked-eye visibility on a moonless night is about 6.0 in a perfectly clear sky, we should be able to see all seven stars that form the constellation. However, we know that on average the limiting magnitude in a city is around 4 at best. This means that we would have trouble finding this constellation in the night sky, as we would only be able to clearly identify the three brightest stars and would only have to guess what the constellation actually looks like.

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The night sky at Roque de Los Muchachos, Canary Islands – Image by Emanuele Balboni – Cosmoedintorni

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Images

Ursa Minor – The Little Dipper & Polaris (John Chumack)