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The Sky This Week, 2016 July 12 - 19

How far the stars?
Saturn, 2016 July 12, 03:16 UT
imaged from the U.S. Naval Observatory
with with the 1895-vintage 12-inch Clark/Saegmüller refractor

The Moon waxes in the evening sky this week, skirting the southern horizon as she passes through the signature stars of summer. Full Moon occurs on the 19th at 6:57 pm Eastern Daylight Time. July’s Full Moon is popularly known as the Hay Moon, Buck Moon, or Thunder Moon, with the latter being particularly appropriate for this time of the year. Luna will be well north of bright ruddy Mars on the evening of the 14th. On the following night she’ll be just three degrees north of Saturn.

This is a good week to try to make sense of the sheer scale of the cosmos as we have a variety of bright objects that we can look at and think of in terms of distance as well as brightness. The first thing we need to do is find a convenient "measuring stick", and fortunately the laws of physics provide us one that’s good for measuring vast distances. Light, the essential "messenger" of all celestial objects, travels at a finite speed that amounts to about 300,000 kilometers (186,000 miles) per second, so if we can measure the time it takes light to reach us from an object we can gauge its distance. When we look at the Moon, we see it as it appeared a just over a second ago. Light from the Sun takes eight minutes to get from there to here. The ruddy glow of Mars is five minutes old by the time it arrives here at earth. Giant Jupiter’s bright glow has taken 50 minutes to cross the gulf of space that separates us. In fact, it was measurements of the timing of eclipses and transits of Jupiter’s moons that led the Danish astronomer Ole Roemer to first calculate light’s approximate speed in 1676. Our last bright planet is Saturn, and the view you’ll get of his wonderful rings tonight will take 77 minutes to get here. Far-flung Pluto, which marked the edge of the solar system when I was in school, is still relatively close-by, just under 4.5 hours at light speed.

Now let’s jump to the rising stars of the summer Triangle in the eastern part of the sky. Bright Vega, highest and brightest star of the trio, is located at a distance that takes light 27 years to cross! Altair, southernmost of the three stars, is even closer, a mere 16 "light years" away. Looking at the last, northernmost member of the triangle one would naturally assume that Deneb would be a similar distance from Earth, but here is where assumptions break down. Vega and Altair are relatively bright "normal" stars, but Deneb is a "blue supergiant" star that shines with the equivalent light of 100,000 suns! Its distance is some 100 times farther away than Altair, so the light you see from it tonight started its journey toward us when Constantine was Emperor of Rome. And we’ve barely ventured into the vast starfields of the Milky Way.

Jupiter is now best seen in evening twilight, and he leaves the sky at around 11:00 pm. You can follow the antics of his four bright Galilean moons during these early evening hours, but it is becoming increasingly hard to spot fine detail on his surface as Earth gives up the heat of the day to the atmosphere

Mars is still easy to spot as evening twilight falls, and you’ll find him close to the meridian at 9:30 pm. His low altitude makes him a difficult object for the small telescope, but his apparent diameter is still larger than it has been for the past several years. A view through the telescope will now show the red planet with a distinctive gibbous phase, and over the course of the next week some of his more prominent surface features will slowly rotate into view.

Saturn is your best bet for a satisfying showpiece in the eyepiece. Like Mars he is at a very low declination, but unlike Mars his bland surface is overshadowed by his dazzling rings wafting serenely in the field of the eyepiece. If you have a small telescope, you’ll be an instant hero in your neighborhood if you give your neighbors a peek at Saturn on the next clear night.