Tonight – September 13, 2016 – Mars presents its smallest phase of the year toward Earth. Did you know Mars shows phases? It does, though not as noticeably as Venus or Mercury, the two plants orbiting inside Earth’s orbit around the sun. You need a telescope and good seeing conditions to glimpse the Martian disk about 85% illuminated in sunshine around now. To the eye alone, Mars appears starlike, though this world tends to shine with a steadier light than the twinkling stars.
By a wonderful coincidence, the moon and Mars momentarily display the same phase on September 13. Neat, huh? And perfectly coincidental. Although the moon goes through the whole range of phases in September (and every month), Mars exhibits nearly the same phase all month long. Look for tonight’s waxing gibbous moon to provide a very good approximation of Mars’ present phase.
Because Mars orbits the sun outside the Earth’s orbit, Mars does not exhibit the whole range of phases, as our moon does.
In fact, only planets that orbit the sun inside of Earth’s orbit – Mercury and Venus – show the complete range of phases.
Planets that orbit the sun outside the Earth’s orbit – Mars, Jupiter, Saturn, Uranus and Neptune – always stay full or close to full, as Mars is doing now with its 85% phase.
Mars’ smallest phase always happens when this world is at quadrature, that is, making an angle with the Earth and sun that is 90 degrees. If you had a bird’s-eye view of the solar system plane, you’d see the sun, Earth and Mars forming that right angle in space now – and at every quadrature – with Earth at the vertex of this angle. See the diagram below.
At east quadrature on September 13, 2016, the day side of Mars covers 84.7% of the Martian disk, showcasing the smallest Martian phase of the year.
Incidentally, when the moon is at east quadrature, or 90o east of the sun, it’s at first quarter phase. So it’s really pretty ordinary, yes? Like Mars, the moon at eastern quadrature climbs highest up in the sky about six hours after the sun soars to its highest point at solar noon (midday).
Yet, unlike Mars, whose phase is conspicuously gibbous at east quadrature, the lunar disk is 50% illuminated at this juncture.
Martian highlights for the year 2016
Feb 7 Mars at west quadrature
Apr 17 Mars starts retrograde
May 22 Mars at opposition
May 30 Mars closest to Earth
Jun 30 Mars ends retrograde
Aug 25 Mars-Saturn conjunction
Sep 13 Mars at east quadrature
Oct 29 Mars at perihelion
At west quadrature on February 7, 2016, the Martian disk was more greatly illuminated than at east quadrature on September 13, 2016 (89.9% versus 84.7%). The difference is due to Mars’ highly eccentric (oblong) orbit around the sun, which causes Mars’ distance to be far from equal at each Martian quadrature.
For instance, at the February 2016 west quadrature, Mars was 1.64 astronomical units (AU) from the sun and 1.31 AU from Earth. In stark contrast, the September 2016 east quadrature finds Mars much closer, at 1.40 AU from the sun and 0.97 AU from Earth.
When Mars is closer at quadrature, the illuminated portion of the Martian disk is smaller; and when Mars is farther away, the illuminated portion of the Martian disk is greater.
Because the Martian quadrature on September 13, 2016 is closer than the Martian quadrature of February 7, 2016, the September quadrature sports the smaller phase (84.7% versus 89.9%).
Bottom line: As darkness falls on September 13, 2016, look for the waxing gibbous moon to simulate the planet Mars’ smallest phase of the year.
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.