Watch Out, It’s Directly Overhead!

The winds are blowing and the temperature is still in the low 50s this morning. Our sweet September summer looks to be ending soon. Although snow isn’t in the forecast yet, temperatures will be near freezing by tomorrow night. I like the bluster and feeling of change in the air. Fall refreshes the senses with a briskness that summer lacks.

The star Deneb is almost directly overhead (at the zenith) for skywatchers in the northern U.S. around 9 p.m. in late September and early October. Altair and Vega form the Summer Triangle asterism with Deneb. Maps created with Stellarium.

Back in August, brilliant Vega, luminary of the constellation Lyra the Harp, lorded over the sky from its perch near the zenith, an imaginary point that’s directly overhead. Lay on your back and you’ll stare squarely at it. You may have noticed that astronomy involves all kinds of imaginary lines and circles: the celestial equator, the ecliptic (path the sun and planets travel through the zodiac) and the meridian to name a few. Then there’s the horizon. At least you can see that one. Anything on its edge has an elevation in the sky of zero degrees. Look halfway between the horizon and the zenith and you’re at an elevation of 45 degrees high (4 1/2 outstretched fists). The zenith itself is 90 degrees high. Since one degree is equal to two full moons side by side, you can squeeze 180 moons between the southern horizon and zenith. And although it sounds improbable, 10 full moons would fit between the five degrees separating the two stars at the end of the Bowl of the Big Dipper.

By late September, Vega is no longer the dominant star at the zenith during the early evening hours. It’s drifted westward to be replaced by Deneb, the brightest star in the Northern Cross. This ceaseless westward drift of the stars is caused by the Earth’s movement around the sun. As we travel in our orbit, we peer out into different sectors of the sky as the weeks and months pass. Think of sitting on one of those merry-go-round horses and looking out into the carnival crowd. As the merry-go-round begins to turn, we look out at a different part of the fairground as we move in a circle round and round. The view repeats every turn so we briefly face the same direction again as when we started. Substitute the Earth for the horse and our orbit for the merry-go-round and you’ve got the picture. Our view of the night sky repeats once a year, so if you look up tonight at 9 p.m. and see Deneb near the zenith, you’ll see it there again at the exact same time next year.

Here you can see the Cygnus is a swan flying south along the Milky Way. Vega is the brightest star in Lyra the Harp and Altair the brightest in Aquila the Eagle.

The Northern Cross is an easy shape to recognize even in moonlight. The constellation name Cygnus actually refers to a swan, which is just as easy to recognize though we don’t often look at it that way. Picture Deneb as the tail of the bird, the cross beams as outspread wings and the bottom of the cross as the swan’s head. See what I mean? In Greek mythology, Cygnus was the god Zeus disguised as a bird flying along the Milky Way on his way to yet another love affair. 

Last week we talked about the curve of the Earth’s surface and how, as you traveled south, new stars would eventually rise up from below the southern horizon. Likewise, as you travel north, stars would pop up from beneath the northern horizon. While Deneb is nearly overhead for the northern U.S., it’s not if you live in Florida or southern California. Miami is 22 degrees of latitude south of Duluth, which means that Deneb is pushed 22 degrees further up and into the northern sky as seen by Miamians (take off sunglasses first, please). The zenith point for southern Florida is midway between Deneb and Altair. Keep going south and Altair would be at the zenith in Caracas, Venezuela. As you travel, so does the sky.

On this map of the Earth, the horizontal lines represent latitude or degrees north and south of the equator. The vertical lines (arcs) are lines of longitude, measured east and west of the prime meridian in England. You can locate any place on Earth if you know its latitude and longitude. Ditto for the sky if you know a star’s celestial coordinates called declination and right ascension.

You can find your personal "zenith stars" by looking up a star’s celestial coordinates called right ascension and declination. The numbers are similar to latitude and longitude on Earth. Declination (latitude) is given in degrees and subdivisions of degrees called minutes. Right ascension (similar to longitude) is given in hours and minutes. For our discussion, we’re just interested in declination. Stars that have a declination the same or close to your latitude will pass through the zenith sometime during the day. Deneb’s declination is +45 degrees so it’s directly on the zenith if you live in say, Central Wisconsin. To find your latitude and longitude, click on either this website or this one and type in your city’s name.