On the evening of October 1, my wife and I were fortunate enough to be sitting by a small lake, watching the stars come out as the sky grew dark after sunset. Some of the first stars out were Vega, Altair, and Deneb. These are the stars of the asterism known as the Summer Triangle.
When I pointed those stars out, my wife commented on how the “Summer” Triangle was so prominent on October 1. And that was true; the Triangle was high overhead, as can be seen in the image below (made with the Stellarium planetarium app, set to display lines for asterisms such as the Summer Triangle, and with the sun about 9° below the western horizon).
And do you know what? Not much has changed in a month. If you go outside after sunset tonight, you will find the Summer Triangle high overhead (below).
In fact, the Summer Triangle is prominent after sunset for much of the year. Below are representations of the sky after sunset for the first of the month of each month of the year, starting with July. They were all made with the Stellarium planetarium app, set to display lines for asterisms such as the Summer Triangle, and with the sun about 9° below the western horizon.
Note how the Summer Triangle is pretty high in the sky after sunset from July all the way through January — six months. It’s not the “Summer” Triangle. It’s the “Half-a-year” Triangle. Contrast it with Orion. Orion is pretty high in the sky after sunset for only February, March, and April.
The cause of this is the changing time of sunset. If we look at the sky at a fixed time, such as midnight, the constellations change from month to month in a very regular fashion, as seen below (this time starting in January).
But “after sunset” — that is, dusk — is not a fixed time. The sun sets a lot earlier in October than in July. The time shown in the July Summer Triangle image is 8:16 PM (standard time); in the October image, it is 6:23 PM. The Summer Triangle is high overhead very late in the evening in July (as measured by the clock, not the sunset), and much earlier in the evening in October, following the regular change of the constellations. If you go out at dusk in both months and look at the stars, you see pretty much the same sky — but of course you are going out at very different times.
The opposite effect is present in the spring as the sun sets later and later. So if the stars seen at dusk don’t change much from July through January, they change quite rapidly from January through July. Orion is prominent after sunset for half the time that the “Summer” Triangle is.
