By Prashant Naik
The adjacent chart shows the night sky over Shillong during the month of September 2020 at 8 pm. Shillong being at 25.5o N Latitude we see mostly northern constellations. The E-W line shows the celestial equator and the line cutting it shows the zodiac line or the sun’s apparent path. The letters indicate the name of constellations. Planet Saturn and Jupiter are visible near overhead.
The prominent constellations visible are Aquila, Cygnus, Lyra (the famous summer triangle), Pegasus, Bootes, Hercules, Draco, Cassiopea, Cepheus, Ursa Major, Ursa Minor, Libra, Aquarius, Sagittarius, Scorpius and Ophiuchus.
The constellation of the month is Sagittarius. The Babylonians, identified Sagittarius as the god Pabilsag (which had wings and a lion’s head). In Greek mythology, Sagittarius is identified as a centaur, half human, half horse. In some legends, the Centaur Chiron was the son of Philyra and Saturn, who was said to have changed himself into a horse to escape his jealous wife, Rhea. Chiron was eventually immortalised in the constellation of Centaurus, or in some version, Sagittarius. The arrow of this constellation points towards the star Antares, the ‘heart of the scorpion’
Of the 88 constellations, Sagittarius probably contains the most abundant and wide-ranging collection of objects. Within its area lie stars, the galactic nucleus, gaseous nebulae, open and galactic clusters – the list is virtually all embracing. It straddles the ecliptic and thus forms one of the zodiacal constellations.
The stars in Sagittarius receive Greek designations totally out of order with their measured apparent magnitude. The brightest star is å, Kaus Australis, a mag 1.81, B9 type at 124 light years. Next is ó, Nunki, mag 2.12, a B2 star at 300 light years, followed by æ, Ascella, a mag 2.61 of A2 spectrum, which is actually a double with mags 3.3 and 3.5, Kaus Meridionalis, is less bright again of mag 2.71 with K2 spectrum and an absolute mag of 0.7, at a distance of 85 light years.
Stars ë, Kaus Borealis is another K2, mag 2.8 at 71 light years distance, followed by ã, Al Nasl, of mag 2.97, a K0 type at 124 light years. Star ç is actually a double with component mags of 3.17 and 10. A triple system, seen as star ð Sagittarii, has mags of 3.7, 3.8 and 6.0. Elsewhere, the constellation displays the Trifid Nebula, M20, a faint and complex gas cloud, the Lagoon Nebula, M8, and the Horse-shoe Nebula, M17. Globular clusters M22, M28, M69, M70, M54, M55 and M75 are to be found as well as open clusters M18, M24, M25, M23 and M21.
New moon will be on 2nd September and full moon on 17th. The autumn equinox that is when the day and night are equal will be on 22nd of the month.