Observing Twilight into Dusk

The minutes after sunset are some of the most beautiful, as the last rays of sunlight have disappeared and the sky begins its daily transition from bright blue to dark night. Looking west after sunset in the early days of Fall, you quickly find Arcturus shining due west, at first looking almost like an airplane that is moving very slowly. But soon you realize that this is a star, the third-brightest in the heavens. It is bright enough to shine clearly through the glow of twilight. The colors of the sky are changing rapidly during twilight, and the brightest stars soon emerge. In late September, you can easily find the fifth-brightest star in the heavens, Vega, directly overhead, guiding you to its neighbors Deneb and Altair in the Summer Triangle. And looking back toward the western horizon where Arcturus is shining, you can soon find the red supergiant star Antares shining low to the left (south-west) of Arcturus.

Later this week, the young Moon arrives in the twilight sky and passes near Antares. Find a nice western horizon, get comfortable, and enjoy the evening show that is pleasant, relaxing and beautiful to see.

Image courtesy of Sky & Telescope Magazine.

The Summer Triangle

One of the brightest objects in the nighttime sky each summer is called the Summer Triangle. Rather than being a single constellation, the Summer Triangle is an "asterism," a grouping of stars that make an interesting pattern but are not themselves a single constellation. The Summer Triangle is, as advertised, a very distinctive triangle of three very bright stars that form a 30-60 right triangle. The three bright stars are Vega, Deneb and Altair. With three stars at magnitude 0 or 1, the Summer Triangle shines through even light-polluted city skies.

If you have darker skies, the Summer Triangle is a quick guidepost to locating some of the most interesting objects you can see with binoculars or a telescope in the Milky Way. The image to the right is taken from the website of the Great Smoky Mountain Astronomical Society. I like their web images because they present a very easy-to-follow guide to the most interesting objects in the part of the Milky Way that passes through the Summer Triangle, including double stars, nebulae, star clusters and the patterns of the three constellations that include the three bright stars of the Summer Triangle: Lyra, Aquila and Cygnus. For another description of the same objects, try this page from the Royal Observatory in Greenwich.

Use your binoculars or a telescope - even if you are in a big city - and see how much of these constellations you can identify and how many of the interesting objects you can find lurking in their midst.