Friday, July 1, 2016

The July Sky

With the arrival of July, the Summer Solstice is still a recent memory, which means that the Sun won't be going anywhere soon. Result: those short nights are going to be sticking around, making for brief, but good times under the stars as the summer sky is, more than any other season, a cosmic picture book.

Cool Constellations
With the arrival of July, the summer sky is at its peak for viewing as all the major sights are now visible without having to stay up half the night. First of all, spring leftovers Hydra (or at least its head end),
Corvus, Cancer, Leo, and Crater will be disappearing for the year. Virgo, highlighted by bright Spica, is also getting very low in the Southwest. Also, the Big Dipper signpost is now obsolete as the last two stops in the chain are gone with only Arcturus (alpha Bootes ) and Spica (alpha Virgo ) still remaining in the sky. Back to the Dipper, it's now pointing downward come nightfall. Onto the summer sky and the cosmic picture book. First up, Corona the crown, with a little imagination, looks like its namesake, or at least a tiara. Moving over, mythological hero Hercules looks somewhat human. Continuing into the Summer Triangle, Lyra looks vaguely like an ancient lyre. Going down, Cygnus is very swan-like and Aquila, with a little imagination, looks like an eagle. The mini constellations Delphinus and Saggita? Yes, they look like their namesakes, too. Now, moving to the South, Libra, though dim, does look like an ancient string scale. Scorpius? Well, using your imagination, the profile can resemble a scorpion. Finally, Sagittarius, one look at it instantly reveals why it is nicknamed the Teapot. Also, there is the Milky Way, which arches high overhead on summer nights and serves as a good measure of how good (or bad) your sky is when it comes to light pollution. By the time the sky starts to brighten, a fall preview in the form of Pegasus, both Pisces, Cetus, Andromeda, Aries, Capricorn, Aquarius, and even Perseus is on tap, too.

Planetary Perceptions
On the planetary front, July marks a month of transition as Jupiter, which has been up for seemingly forever, is becoming a twilight object as early July offers the last opportunity to catch the planetary king under a truly dark sky. By month's end, Jupiter is strictly twilight. Additionally, for the first time in months, there are no predawn planets visible as both Mercury and Venus are in twilight, with the former making a poor twilight appearance late month and the latter making an even poorer showing at the same time. July also makes for good times when observing Mars and Saturn as both are just about due South as the sky gets dark. Unfortunately, this being summer, the planets are not all that high thanks to the flat ecliptic plane. As a final item of note, Mars is finished retrograding, which means that it will once again be converging with, and eventually passing the slower Saturn.

Tonight's Sky for July 1: Moon at Perigee
Tonight, the Moon is about as big as it will ever get thanks to the fact that the Moon is at perigee, a point in its orbit that is closest to Earth.

What many people may not realize is the fact that the Moon (and all other celestial bodies) do not orbit their parent bodies in circles, but ellipses, which are slightly elongated circles. Result: any given day of an orbital period, any orbiting body will be at a slightly different distance from its parent body. As for the Moon, this variance in orbit amounts to about 20,000 miles.

As for tonight, the Moon will be about as close to Earth as it is going to get. When it comes to practical implications, the difference will be hard to notice with the naked eye to all but an experienced observer but, in a telescope, the difference will be obvious

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