When knights went into battle, they depended not only on their sword, but on their shield as well. While we do not carry shields around with us, there is one about halfway up in the sky above our southern horizon. Scutum, The Shield, is 84th out of the 88 official constellations in size, covering only 109 square degrees.
Renaissance astronomers had a sky that was filled with constellations from Greek mythology as listed in Ptolemy’s Almagest, the (at the time) authoritative book on astronomy. Claudius Ptolemy lived in Alexandria, Egypt under the Roman Empire of the second century. The Almagest included tables for computing the positions of planets, a star catalog and a listing of constellations. The constellation list was derived from Greek astronomer Hipparchus’s list of 48 constellations.
This list did not cover the entire sky. There were many areas that were not part of these 48 constellations and there were no constellations in the southern hemisphere where Hipparchus could not see because they were below his southern horizon. Renaissance astronomers began inventing new constellations to fill in the empty areas in the sky.
Polish astronomer Johannes Hevelius created the constellation Scutum Sobiescianum (Shield of Sobieski) to memorialize Polish King John III Sobieski’s victory in the Battle of Vienna in 1683. Hevelius took the brightest stars in this constellation from neighboring Aquila when he drew the new constellation. The brightest of these stars is Alpha Scuti shining at magnitude +3.9, a somewhat faint star for the brightest star in a constellation.
Such a small constellation has few really interesting objects, but comets are visitors that travel through many constellations as they make their rounds in our Solar System. Most comets have a tail, a feature that sets them apart from most other astronomical objects. Perhaps you heard about or saw pictures of a new comet that graced our skies in July, Comet 2020 F3 (NEOWISE).
A comet is a ball of frozen dust and gas, a “dirty snowball” as hypothesized by American astronomer Fred Whipple in the 1950s. They are normally completely solid when they are far from the Sun. The comets we see generally have orbits that keep them in the outer Solar System most of the time, only occasionally coming into the inner Solar System. As they approach the Sun, they warm up. The water-ice and other ices sublimate, changing directly from the frozen state to the gaseous state, without ever becoming liquid.
In some comets the gas simply drifts off, carrying the dust that it had been trapping with it. In other comets, the gas and dust stream out as jets from underground reservoirs. Since most comets rotate, these jets create spirals of dust and gas around the heart of the comet itself, called the nucleus.
The gas and dust drifts along with the nucleus, forming a cloud around it, called the coma. The coma is usually spherical surrounding the nucleus, especially when it is still far from the Sun, but not so far that the nucleus is still frozen solid. As it gets closer to the Sun, dust and gas is released more rapidly.
Charged particles stream outward from the Sun in such numbers that they are referred to as the solar wind. These particles strike the gas coming out of the nucleus and push it directly outward from the Sun, separating it from the dust that remains along the comet’s orbit. With a bright comet, this results in two tails emerging from the comet.
The gas tail is straight, pointing directly at the Sun. It usually has a blue color, though if the comet has more sodium that the usual comet, the blue may be tinted with green. The dust tail usually is a white color, reflecting the light from the Sun. Since it follows the orbit of the comet, the dust tail usually is curved following the comet’s orbital path.
Comets do not “shoot across the sky”. They move slowly against the background stars as they travel in their orbit. Every night, the comet, along with all the other stars rises or sets. Since the comet is brightest when it is near the Sun, bright comets are usually found in the east before sunrise or in the west after sunset. Comet NEOWISE is the brightest comet visible in the Northern Hemisphere in the past 23 years. Perhaps another one will brighten our sky in the near future.
The Planets for September 2020.
As the month begins, Mercury appears above the western horizon as it emerges from the far side of the Sun. It will spend the entire month in Virgo, moving from the far western end of the constellation to the far southeastern end. The Messenger of the Gods will slide southward along the western horizon, only about five degrees up. At midmonth, Mercury’s disc is 5.6 seconds-of-arc across and it will be 80 percent illuminated.
Saturn and Jupiter are still in eastern Sagittarius moving westward at the beginning of the month. Jupiter stops on Sept. 12 and turns back eastward. Saturn continues westward for another seventeen days before it too turns eastward. They are about eight degrees apart, 31 degrees above the south-southeastern horizon as it gets dark, setting a little after 4 a.m.
At midmonth, Jupiter has a disc that is 42.4 seconds-of-arc across, shining at magnitude -2.5. Saturn is magnitude +0.4 with a disc that is 17.5 seconds-of-arc across. Its rings are tilted 22.8 degrees downward with the northern face showing and they are 39.8 seconds-of-arc across.
Mars rises around 9 p.m. on the eastern horizon and it is 39 degrees above the west-southwestern horizon as it gets light. Shining at magnitude -2.2, Mars’ disc is 21.1 seconds-of-arc across at midmonth. The God of War is moving eastward until Sept. 9 when it turns around and heads westward in eastern Pisces.
The brightest planet in our sky shines at magnitude -4.1 as it rises around 3:30 a.m. Venus will be 35 degrees above the eastern horizon as it gets light. During the month, it moves from eastern Gemini, through Cancer, and into southwestern Leo. The Goddess of Love’s disc is 17.3 seconds-of-arc across and it is 66 percent illuminated.
Day and night are of equal length on Sept. 22, marking the Equinox. On this day, the Sun crosses the celestial equator traveling southward. In the Northern Hemisphere, the season of astronomical autumn begins on that date. So, start unpacking your winter clothes and "keep watching the sky"!
An amateur astronomer for more than 45 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.