Lunar sunrise and the 1st quarter moon

Image at top via U.S. Naval Observatory

Enjoy sunrise on the moon tonight! The February 3, 2017 moon will be at or near its first quarter phase. At quarter moon, the moon’s disk appears to be one-half illuminated in sunlight and one-half immersed in the moon’s own shadow. The moon has a day side, and a night side, just as Earth does. On February 3, 2017 we on Earth are seeing half the moon’s day side, or a quarter moon.

When the moon is waxing from new moon to full moon, the lunar terminator – the shadow line dividing the lunar day from the lunar night – shows you where it’s sunrise on the moon. If you had a telescope and could watch over several nights, you could see this line of sunrise creep across the moon’s face, bringing more daylight to the Earth-facing side of our companion world.

Meanwhile, if you could see the far side of a first quarter moon, the terminator line – line between light and dark – would show you where it’s sunset on the moon.

Now imagine waiting a week or two, until the moon has passed its full phase and is waning from full moon to new moon. On a waning moon, the lunar terminator shows you where its sunset on the moon. Meanwhile, if you could view the far side of the moon at this juncture, you’d once again see the terminator as the sunrise line.

Along the lunar terminator line – sunrise or sunset line on the moon – shadows are long, just as sunrise/set shadows are on Earth. On June 10, 2011, NASA’s Lunar Reconnaissance Orbiter captured this dramatic sunrise view of the moon’s Tycho crater. Image via NASA.

Want the exact time of the moon’s first quarter phase? It occurs on February 4, 2017 at 4:19 UTC. At United States time zones, that means the moon reaches the exact first quarter phase on February 3 at 11:19 p.m. Eastern Time, 10:19 p.m. Central Time, 9:19 p.m. Mountain Time, 8:19 p.m. Pacific Time, 7:19 p.m. Alaskan Time and 6:19 p.m. Hawaiian Time. Generally speaking, a first quarter moon climbs highest up for the day about six hours after the noonday sun. It sets around the middle of the night.

Image via Wikipedia. An example of a planet, such as Mars, at eastern and western quadrature,. Earth is at the vertex point as Mars, Earth and sun form a right angle in space.

Example of a planet, such as Mars, at eastern and western quadrature. Earth is at the vertex point as Mars, Earth and sun form a right angle in space. Image via Wikipedia.

At first quarter, astronomers say the moon is at eastern quadrature. It is 90o east of the sun on the sky’s dome. In contrast, a last quarter moon is synonymous with a moon at western quadrature, when the moon is 90o west of the sun.

The moon or any celestial body is said to be at quadrature whenever the sun, Earth and that celestial body (in this case, the moon) make a right angle in space, with Earth at the vertex of this 90o angle.

If you had a bird’s eye view of the moon’s orbital plane at its first and last quarter phases, you’d see the moon, Earth and sun making a 90o angle in space. However, at the moon’s waxing and waning crescent phases, this angle is less than 90o from the sun; at the moon’s waxing and waning gibbous phases, the moon is more than 90o from the sun in Earth’s sky.

At full moon, the moon is 180o from the sun, with the moon, Earth and sun making a straight line in space. The diagram below helps to illustrate:

Image via NASA. Not to scale! The moon's distance from Earth is equal to about 30 Earth diameters, and the sun's diameter spans about 109 Earth diameters, which is more than 3 times the Earth-moon distance. Last but hardly least, the Earth's distance from the sun spans about 109 sun diameters.

Not to scale! The moon’s distance from Earth spans about 30 Earth-diameters. Sun’s diameter spans about 109 Earth-diameters. Earth’s distance from the sun spans about 109 sun-diameters. Image via NASA.

The upcoming full moon will sweep right through Earth’s faint penumbral shadow to stage a penumbral lunar eclipse on the night of February 10, 2017.

Bottom line: Enjoy the moon on February 3-4, 2017 as it exhibits its first quarter phase. And think about watching a sunrise or sunset on the moon.

Read more: Understanding moon phases by Deborah Byrd

Bruce McClure