My previous blog post was five months ago. Sorry, it’s been a busy spring and summer! I’ll try to get back to posting about once a month. Also, I promised in my last post to write about Uranus, Neptune, and Pluto. I’m going to delay that until my next post, because August 2018 is a quite a good month for observing four bright planets: Venus, Jupiter, Saturn, and Mars.

Below is a map of the complete “sky above” as of 9 PM EDT on August 1st. The periphery of this map is the horizon, and the center of the map is our zenith: the point straight overhead. (Hint: If the print in the map is uncomfortably small, try using a magnifying glass.)

Venus is in the constellation Virgo, low in the western sky. Jupiter is somewhat higher in the sky: it’s in the faint constellation Libra above the south-southwest horizon. Saturn is low in the south-southeastern sky, just above the spout of the teapot pattern in the constellation Sagittarius. Mars is just rising in the southeast; it’s in the faint constellation Capricornus.

[singlepic id=3056 w= h= float=none]

Later in the evening of August 1, Venus and eventually Jupiter will set, while Saturn and Mars will get higher in the sky. These planets and all the stars rotate in counterclockwise circles around the North Star, which you can find on the map. The North Star is labeled Polaris on the map. It is not quite halfway from the North horizon to the zenith.

The two maps below show the stars and planets as of 9 PM EDT on August 16 and August 31. As you can see by comparing all three maps, the 9-PM positions of the constellations gradually shift. This is because our clocks are based on the Sun, which moves steadily eastward through the constellations of the zodiac. If you look carefully, you can also see from the maps that the planets slowly “wander” among the constellations. (The name “planet” comes from the Greek word for “wanderer.”) Venus and Mars change apparent locations among the stars more rapidly than Jupiter, and Saturn wanders hardly at all during the month of August.

[singlepic id=3057 w= h= float=none]

Can you find the crescent moon on the August 16th map? It’s between Saturn and Jupiter on the sky, but much closer to Jupiter. It’s a relatively fat crescent, because it’s only two days away from first quarter phase, which occurs on the 18th. If you look at the 9-PM sky on successive nights around the middle of the month, you will see that the Moon drifts eastward quite a bit (about 13 degrees) from night to night among the stars. The Moon makes a complete circuit around the zodiac every 27.3 days. The month of the phases is longer (about 29.5 days) because to complete the phase cycle the Moon must catch up to the Sun, which is also moving slowly eastward (about 1 degree per day) along the zodiac.

[singlepic id=3058 w= h= float=none]

Happy planet hunting for August!

(Map credit: Interactive Sky Chart, which can be accessed via the Sky and Telescope website,)

--Bill Ingham