Fall is almost upon us as the month begins. The weather becomes a little more comfortable, and evening comes a little earlier. With this change comes slightly better chances for clear skies and good seeing conditions. This is a great time of the year for stargazers!
There are just two new constellations to introduce this month: a northern circumpolar constellation and a zodiacal constellation A planosphere or smartphone app like Google Sky Map can help determine where to look to find each constellation, and a Wikipedia link is provided to further aid in identifying each constellation’s appearance.
Cepheus (The King of Aethiopia)
This a fairly bright and recognizable circumpolar constellation at my latitude of about 39 degrees. It is a house-like asterism of fairly bright stars that can be easily found south and east of Polaris in September’s evening skies. A circumpolar object (star, constellation, deep space object) is one that never sets below the horizon during the Earth’s daily rotation. Any point in the sky that has a right ascension (degrees of separation from the celestial equator) greater than the observer’s latitude (degrees of separation from the terrestial equator) will never set below the horizon. If for some reason the Sun ceased to illuminate the sky, a circumpolar object could be seen to circle Polaris once in about every 24 hour period. Cepheus, for instance, is oriented with the top of the house-like asterism nearest Polaris. In the September evening sky, Cepheus is high above Polars, and the “house” appears to be upside down. Over the course of 24 hours, Cepheus will circle Polaris. In six hours, the “house” is horizontal with the top pointing west, in 12 hours it appears upright, but below Polaris, in 18 hours it appears vertical again, this time with the top pointing east. Six months from now in March, Cepheus’ orientation at dusk is the same upright appearance as Cepheus’ 12-hour position in September.
Capricornus (The Goat)
Capricornus is one of the 48 constellations cataloged by Potolemy in the 2nd century BC, and that is still recognized by modern astronomers. Sagittarius, last month’s zodiacal constellation, is a jumping off point for finding Capricornus, which can be found just off of its eastern flank. The combined effect of its dim stars and low position on the horizon makes this a difficult constellation to pick out in a light-polluted sky. It’s alpha star, Deneb Algedi, is just brighter than magnitude 3, and can be seen under most reasonable conditions.
SOLAR SYSTEM OBJECTS
The Sun. The autumnal equinox occurs when the Sun crosses the celestial equator on September 23rd, and thus astronomical autumn officially begins. The days will continue to get grow shorter at a faster rate until the equinox occurs, at which time the length of day and night are approximately equal. After the equinox, the rate at which the days begin to shorten and the nights grow longer and begins to slow down.
|September 2||First Quarter|
|September 8||Full Moon|
|September 15||Last Quarter|
|September 20||Conjunction with Jupiter|
|September 24||New Moon|
|September 27||Very close conjunction with Saturn|
|September 29||Conjunction with Mars|
Mercury‘s evening apparition around September 23rd will be among the worst of the year. Look for it low on the western horizon, above and to the left of the sunset point just after sunset.
Mars remains in Scorpius, and is the bright reddish colored object appearing in the southwest after sunset. It is located near Antares, a reddish star of nearly equal brightness. Be sure to look for this pair!
Jupiter can be found shining brightly above the eastern horizon shortly before dawn, higher this month than last.
This apparition of Saturn is nearly over. Look for it very low on the western horizon just before sunset. There will be a pretty conjunction with a fingernail crescent Moon on September 27th. By month’s end, Saturn will be visible only to the most determined observers as it becomes increasingly lost in the Sun’s glare.
September 8th – Supermoon
Another supermoon, also known as a perigean moon, occurs on the event of September’s full moon. A supermoon is said to occur when the Moon is full at the same time it is near perigee, or when it is at its closet approach to Earth during its orbit.
© 2014 James R. Johnson.