Tonight – March 12, 2017 – look eastward as darkness falls to see the brilliant, full-looking moon illuminating the nighttime sky. By early evening, watch for the dazzling planet Jupiter to follow the moon into the nighttime sky. The moon and Jupiter will climb upward during the evening hours, to reach their high point for the night somewhat after midnight. Thereafter, the moon (and Jupiter)will sink westward during the wee morning hours, to adorn the western sky at dawn.
The moon turns full today, on March 12, at 14:54 UTC. At North American time zones, that means the full moon occurs during the morning daylight hours today, on March 12, when the moon is below the North American horizon. Tonight, it’ll actually be a waning gibbous moon that lights up the evening of March 12 in North America, although the moon will still look plenty full to the eye.
Check out the worldwide map below. The shadow line running through the Middle East and to the right of Africa and Madagascar depicts sunset March 12. In other words, the moon turns exactly full as the sun is setting in Saudi Arabia. Therefore, it shouldn’t come as too much of a surprise that the full moon rises and that the sun sets at nearly the same time in Riyadh, Saudi Arabia (moonrise: 5:58 p.m. local time; sunset: 6:00 p.m. local time) on March 12, 2017.
It’s not often that you’re in the part of the world where the moon turns precisely full right at sunset. Given a level horizon, as at sea, we might expect the moon to rise and the sun to set in concert if this were the case.
However, the full moon is only truly opposite the sun during a total lunar eclipse, when the moon sits smack-dab in the middle of Earth’s dark umbral shadow. Depending on which of the year’s 12 full moons were referring to, the full moon can be anywhere from 5o north to 5o south of the ecliptic (Earth’s shadow). So, for this reason, the full moon rises at the vicinity of sunset but not necessarily right at sunset.
Let’s presume a total lunar eclipse happening exactly at sunset in our part of the world. Given a level horizon, as at sea, perhaps we’d expect to see the solar disk setting in the west while the eclipsed lunar disk is rising in the east. At any rate, we probably wouldn’t expect to view both the sun and eclipsed moon to be above the horizon at the same time.
Reportedly, the ancient astronomer Cleomedes saw BOTH the sun and moon above the horizon during a lunar eclipse. He had an explanation for the unexpected surprise, correctly surmising that the Earth’s atmosphere refracts (bends) light, raising the sun and moon above their true geometrical positions. When at or near the horizon, atmospheric refraction lifts the sun or moon about 1/2 degree above their true geometric position. By the way, quite by coincidence, the solar and lunar disks both span 1/2 degree of sky. Therefore, when we see the bottom limb of the sun or moon sitting on the horizon, that means it’s actually the top limb of the sun or moon that’s touching the horizon geometrically.
Atmospheric refraction also accounts for why we get more than 12 hours of daylight on the day of the March 20th equinox.
This March full moon counts as the third of three full moons in between the December solstice and the March equinox. Watch it shine throughout the night, as the season’s final full moon is a welcome harbinger of spring for the Northern Hemisphere, and the Southern Hemisphere’s Harvest Moon.
Read more: Equal day and night on equinox
About The Author:
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.