The best times for viewing stars and planets in September 2019 are the beginning and ending days of the month. A new moon occurs on August 30 and again on September 28 when Earth’s moon is so close to the Sun that they both set at almost the same time. Skies will be darker at these times. In contrast, a full moon occurs on September 13, making it more difficult to see fainter objects.


September is a good time to see the Summer Triangle, an asterism of three of the brightest stars in the sky. An asterism is a group of stars that are associated in some way but are not considered to be a constellation.


Even when the moon is full this month, these stars will lie to the north of the moon and should still be visible. Look directly overhead in the evening sky and a bit to the north to see the lopsided triangle formed by Altair, Deneb and Vega. These three are the brightest stars in their respective constellations.


Altair is closest to overhead and its distance to either Vega or Deneb is about three hand spans. Although this is called the Summer Triangle, it is visible during the year from the early morning in spring to the early evening in January. The name Altair comes from the Arabic meaning “flying eagle” and is in the constellation Aquila the eagle.


Deneb is the tail of the swan in the constellation Cygnus. Vega is in Lyra the lyre. The name Vega implies “swooping” from the Arabic word “waqi”. This harks back to ancient times when people thought of the shape of this constellation as a vulture rather than a lyre.


Stars that appear together in the sky are not always close to each other and this is the case with the members of the Summer Triangle. Altair is about 17 light years away and Vega is 25 light years from the Earth.


Deneb is a great deal farther away. Its distance is not accurately known because it is so far away. The second analysis of the Hipparcos satellite data gives a distance of about 3200 light years. Even so, Deneb appears roughly the same brightness as the other two stars.


How can this be? Deneb is about 200 times the diameter of our sun and is burning through its nuclear fuel very rapidly. This makes it about 200,000 times more luminous than our sun and a close match for the brightness of Altair and Vega as seen from Earth. To put this into time perspective using the distance mentioned, the light you see tonight from Deneb left that star in about 1180 B.C., roughly the time of the collapse of the Hittite empire.