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Moon Phase Calendar

Choose the year: 2011, 2012, 2013, 2014, 2015, 2016,
2017, 2018, 2019, 2020, 2021, 2022

The moon's phase depends on the direction sunlight hits it. Hold up a tennis ball when the sun is near the horizon. Turn so the sunlight hits the ball from different directions and watch the "phase" change. The moon's phase is not caused by the earth's shadow. This is a common misconception.

If you want to see the moon's craters with binoculars or a telescope, the best place to see them on the moon is near the "terminator," the curved line dividing the dark and illuminated sides of the moon's disk. Features near the terminator cast long shadows making mountains and valleys clearly visible. As the month progresses the terminator sweeps across the moon, a little each night, highlighting mountains and craters in different areas. At full moon there are no shadows, so the craters don't stand out well. On the other hand the reflective properties of different materials on the moon's surface become more apparent. During full moon look for "rays," the splash patterns of material ejected from craters when they were formed by the explosive impacts of giant meteorites.

Deep sky observers usually want to observe when the moon is not in the sky because its brightness "washes out" the rest of the sky, making faint celestial objects difficult to see. Go out at full moon and notice how few stars are visible compared to a dark moonless night. The best time for a full night of deep sky observing is near new moon, indicated by dark disks on the Moon Phase Calendars. The moon is in its "new" phase when it is close to lining up with the sun in the sky. Since the sun and moon appear near each other at this time, they rise together and set together. That means when the sun sets the moon sets also, leaving a moonless sky all night long.

The sun is about 400 times as far away as the moon, so when they appear to line up in the sky the moon is actually between the earth and the sun. Sunlight lights up the moon from the far side and the earthward side of the moon is dark. In other words, we really don't see the new moon at all! It is up only in the daytime and the side facing us is black. There is nothing to see. The only time we can actually see a new moon is when the alignment is precise and the moon covers up the sun. This is called a solar eclipse. The black disk that hides the sun is actually the new moon.

The moon moves about 13° eastward each night. A few days past new moon, the moon appears as a thin crescent in the west after sunset. The edge of the moon facing the setting sun (the right edge, if you live in the northern hemisphere or left edge if you live "down under") is the side that is bright. When the moon is thin you can see some features on the dark side (especially with binoculars or a telescope) because it is being illuminated by "earthshine": light from the sun that reflects off the earth and back onto the dark side of the moon. People on the moon would see a bright "full" earth lighting their moonscape during this phase.

About 7 days after new moon comes first quarter. It looks like a "half moon" at this time, but it is called first quarter because it is one quarter of the way through its monthly cycle. The first quarter moon is at right angles to the sun: it is high overhead when the sun is setting on the western horizon. The bright half of the moon faces west, toward the setting sun. The first quarter moon sets near midnight leaving the second half of the night dark. Beyond first quarter, when the moon is more than half illuminated its phase is called "gibbous."

The full moon is opposite the sun in the sky. It rises over the eastern horizon exactly as the sun is setting on the western horizon. If you see the moon rise a little before sunset you know it is not quite full. If it rises a little after sunset it is a little past full. The full moon brightens the whole sky all night, so this is when deep sky observers catch up on their sleep!

You might expect that the full moon is twice as bright as first quarter (the so-called "half moon"), but actually it is about 16 times as bright! This is because the moon is littered with tiny glass beads. Whenever an asteroid or meteorite hits the moon at high speed, forming a crater, the energy of the impact is enough to melt the rock near the impact site and create a splash. Tiny droplets of the molten rock solidify into small glassy marbles that rain back down on the moon's surface. Just as the beaded coating on a stop sign reflects headlights preferentially back toward the source of the light, the moon's beaded surface reflects sunlight back toward the sun. When the moon is full, the earth is nearly in line with the sun, so we catch the backscattered light and we see a dramatic increase in the moon's surface brightness. Look for this effect!

The moon looks much larger rising than when it is high in the sky, but this is just an illusion. Try measuring the moon by holding a ruler at arms length and marking the size of the moon with your thumb. You will get the same results when it is near the horizon or later the same night when it is high in the sky, and you will be amazed how small your measurement will be! An aspirin tablet at arms length will cover the moon!

After full moon the moon rises later and later each night, giving you increasing amounts of dark time before moonrise. The third quarter moon rises near midnight and is overhead at sunrise, leaving the first half of the night dark for observing. Its bright side faces east toward the rising sun.

[The Moon Phase Calendars were computer generated with the program Deep Space.]