While giving a tour of the sky, I am often asked how I know which "stars" in the sky are planets and which are real stars. The short answer is that planets are always on the go and are thus easy to spot. The word "planet" originates from the Greek word for "wandering star" and as suggested, the planets are not fixed against the background of stars in the sky but wander from place to place. Only five planets are bright enough to be easy to spot in the night sky. Because two of them orbit the Sun inside the orbit of Earth (Venus and Mercury), they move very quickly across the sky, and we only see them shortly after sunset or before sunrise. The three outer planets (Mars, Jupiter and Saturn) are visible throughout the night sky and move more slowly against the background of stars in the sky. Much like the ancient astronomers, I have come to know the basic constellations in the sky and can quickly tell when a planet is interloping in the space of a constellation. With a little investment of time, you can do this too.
The early part of this summer will be a wonderful time to get to know the planets. Venus remains a bright evening object through June and slowly fades in July. Saturn too remains visible in the evening but also fades by the end of July. And Jupiter is just beginning its reign over the summer sky as it is visible immediately after sunset in the southeastern sky. Saturn and Venus will make a remarkable pair on July 1st just on the edge of the constellation Leo. Mark your calendar for that event! If you watch over several weeks, you will notice how much change there is in the position of Venus compared to Gemini (where it is now) and Leo (where it will be in a few weeks).
If you buy yourself a good-sized star chart (10 to 12 inch), or if you buy a quality astronomy magazine such as Night Sky, Astronomy or Sky & Telescope and tear out the monthly star chart, you can get to know a few bright constellations that dominate the spring and summer sky such as Leo (where you will find Saturn and Venus), Virgo, Scorpio (where you will find Jupiter), and Sagittarius. Once you get to know these, you will be able more easily notice where the planets fit in and how they move against the background stars. By the end of summer Mars will be visible in the morning moving through Aries toward Taurus.
Make yourself a promise to learn a few constellations this summer and you'll find yourself more at home and comfortable with the night sky. And you will start to discover just where the planets are each night.
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