Tonight – June 17, 2016 – you can’t miss the almost-full waxing gibbous moon at nightfall. This evening’s moon shines in between the red planet Mars and the ringed planet Saturn (of course, you won’t see Saturn’s rings without a telescope). Mars outshines Saturn, and Saturn, in turn, outshines nearby Antares, the brightest star in the constellation Scorpius the Scorpion.
We expect this bright celestial triangle of lights formed by Mars, Saturn and Antares to withstand tonight’s drenching moonlight. Use the moon to find the trio tonight, and then watch them in the evening sky for several months to come.
Because Mars and Saturn are planets, these worlds shine by reflecting the light of the sun. Antares is a star, so it shines by its own light. Planets tend to shine with a steadier light than the twinkling stars. See if this general rule holds true for you tonight.
Mars, the fourth planet outward from the sun, resides at a mean distance of 1.524 times the Earth’s mean distance from the sun. Astronomer’s often call the sun-Earth distance the astronomical unit.
Saturn, the sixth planet outward, resides at a mean distance of about 9.582 astronomical units. A planet’s mean distance from the sun can also be regarded as the planet’s semi-major axis.
Let us share a cool trick with you. Once you know a planet’s mean distance (semi-major axis) in astronomical units, you can figure that planet’s orbital period in Earth years with this easy-to-use formula:
Orbital period2 = mean distance3
Let’s find out Mars’ orbital period in Earth-years:
Orbital period2 = distance x distance x distance
Orbital period x orbital period = 1.524 x 1.524 x 1.524
Orbital period x orbital period = 3.5396
Orbital period = 1.88 Earth-years
We’ll let the inquiring reader figure out Saturn’s orbital period. Remember, Saturn’s mean distance (semi-major axis) = 9.582 astronomical units.
Bottom line: Tonight – June 17, 2016 – let the waxing gibbous moon show you the planets Mars and Saturn, plus the star Antares.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.