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Full-looking moon edges toward Jupiter

Tonight – March 12, 2017 – look eastward as darkness falls to see the brilliant, full-looking moon close to the horizon. By early-to-mid evening, or around 8 to 9 p.m. local time (9 to 10 p.m. daylight saving time), 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. Afterwards, the moon (and Jupiter) will sink westward, to adorn the western sky at dawn.

The moon turns full 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 North American evening sky, although the moon will still look plenty full to the eye.

Check out the worldwide map below of the day and night sides of Earth at the instant of full moon (2017 March 12 at 14:54 UTC). 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 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.

To find out when the moon rises and the sun sets in your part of the world, click here and remember to check the moonrise and moonset box.

Worldwide map via the US Naval Observatory. Day and night sides of Earth at the instant of the March full moon (2017 March 12 at 14:54 UTC). The shadow line at left depicts sunrise on March 12, and the shadow line at right represents sunset on March 12.

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 in this scenario.

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 this year’s 12 full moons we’re referring to, the full moon can swing 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.

Want to know how far north or south the moon is from the ecliptic (Earth’s orbital plane) right now? Or for any chosen time and date? Click here to find out the moon’s ecliptic latitude.

Let’s presume a total lunar eclipse is happening exactly at sunset in our part of the world. Given a level horizon, as at sea, perhaps we’d expect to see half the solar disk above the western horizon and half the lunar disk above the eastern horizon. We probably wouldn’t expect to view both the sun and the totally eclipsed moon 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 rarely seen phenomenon, which is called a selenelion or selenehelion. Cleomedes correctly surmised that the Earth’s atmosphere refracts (bends) sunlight and moonlight, 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 upward. Quite by coincidence, the solar and lunar disks both span about 1/2 degree of sky. For reference 1/2 degree = 30 arcminutes or 30′.

Therefore, when we see the bottom limb of the sun or moon sitting on the horizon, that means it’s pretty much the top limb that’s touching the horizon geometrically.

Most almanacs define sunrise or sunset as that instant when the top edge of the sun touches the horizon. Geometrically, however, the sun’s upper limb is about one sun-diameter below the horizon at sunrise or sunset. Image via Wikipedia.

Atmospheric refraction also accounts for why we get more than 12 hours of daylight on the day of the March 20th equinox.

Bottom line: On March 12, 2017, look eastward as darkness falls to see the brilliant, full-looking moon illuminating the nighttime sky. Jupiter will follow the moon into the nighttime sky by early to mid-evening.

Read more: Equal day and night on equinox

Bruce McClure