For many, recurrences are reassuring. We enjoy the cycle of the seasons and the sun's daily round of the sky. Natural rhythms helped to order life and events for our ancestors who used the positions of the sun, moon and stars to determine the best times to plant or hold important celebrations.

But 24/7 lighting has diminished the stars, while mobile phones have blurred the difference between day and night. The sky and its rhythms, once essential, have become background. Deep down I believe we crave these cycles. They help us create and maintain a bond to the natural world, which is critical to our mental well-being and by extension to our physical health.

To see the newly-emerging lunar crescent, look low in the western sky tonight (April 13) starting about a half-hour after sunset. (Stellarium)
To see the newly-emerging lunar crescent, look low in the western sky tonight (April 13) starting about a half-hour after sunset. (Stellarium)

Starting Tuesday, April 13, a new celestial cycle begins in the evening sky. That's when the moon returns in the west at dusk as a pointy crescent. Whether you live in Rural, Wisconsin (yes, an actual town) or downtown LA, following the moon's movement across the sky and its changing phase are there for all to see.

This diagram shows the moon's orbit and its changing phases. The sun is out of the frame to the right. Half of the moon (shown in white) is always lit by the sun as seen from afar, but the amount we see varies according to the angle the moon makes to the sun. When it passes between the Earth and sun at new phase, it's invisible. As the moon's angle to the sun increases so does its apparent separation. At first quarter phase, half the moon is lit up. At full moon, we see the entire nearside of the moon in sunlight. (Public domain)
This diagram shows the moon's orbit and its changing phases. The sun is out of the frame to the right. Half of the moon (shown in white) is always lit by the sun as seen from afar, but the amount we see varies according to the angle the moon makes to the sun. When it passes between the Earth and sun at new phase, it's invisible. As the moon's angle to the sun increases so does its apparent separation. At first quarter phase, half the moon is lit up. At full moon, we see the entire nearside of the moon in sunlight. (Public domain)

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At new moon phase, the moon lines up almost directly between the Earth and sun. Not only is the new moon invisible in the solar glare, but no sunlight falls on the side that faces us. There's literally "nothing to see here." Only at those infrequent times when the moon passes squarely between the two orbs in solar eclipse is the moon visible as a black circle slowly biting into the sun.

The moon orbits the Earth at an average speed of 2,290 miles an hour (1 km/sec) and moves approximately its own diameter (1/2°) every hour to the east. This adds up to to 13° a day or a little more than one balled fist held at arm's length against the sky. The stars travel from east to west because of Earth's spin, but the moon moves exactly opposite, from west to east. The reason it "goes along for the ride" and sets in the west like the stars do is because it's eastern motion isn't fast enough to overcome the speedier stars.

You can easily see the moon's fist-a-day progress especially during the next few nights because it will travel almost directly upward from the western horizon, higher and higher into the evening sky. I like to pick a bright reference star or two to gauge its nightly creep. By the way, you don't necessarily have to wait until the next night to see the moon advance. When it happens to be near a bright star or planet, keen-eyed observers can detect its movement in just an hour or two.

As the stars rise in the east and set in the west the moon slowly travels in the opposite direction some 13° (one fist) per night. This map shows the moon's nightly travels to the east, starting as a slim crescent on April 13th and ending with the full moon on the 26th. (Stellarium with additions by the author)
As the stars rise in the east and set in the west the moon slowly travels in the opposite direction some 13° (one fist) per night. This map shows the moon's nightly travels to the east, starting as a slim crescent on April 13th and ending with the full moon on the 26th. (Stellarium with additions by the author)

As the moon travels to the east between now (April 13) and full moon on April 26, it distances itself from the setting sun, gradually working its way from the western sky to the southern sky and finally to the east. At full moon it lies 180° opposite the sun (stick your arms out again to prove it!), rising at sunset and remaining visible the entire night.

Not only is the moon always on the move, but as it circles the Earth its angle to the sun changes slowly and continuously. This in turn causes its phase — the amount of the moon we see illuminated by sunlight — to wax and wane. At new moon, it makes a near-0° angle with the sun because it's in the same line of sight. But just a day later the moon has slid 13° to the sun's left (east), exposing its sunlit edge as a skinny crescent. As the moon's angle and apparent distance from the sun increases, so does its phase, widening from a sliver to a banana.

The waxing phases of the moon from new to full are shown at top. I've also created the corresponding phases on a subject's face by changing the position of a studio light. In a similar way, the moon's changing angle to the sun (the big studio light in the sky) causes the lunar phases. (Top: NASA photos / Bottom: Bob King)
The waxing phases of the moon from new to full are shown at top. I've also created the corresponding phases on a subject's face by changing the position of a studio light. In a similar way, the moon's changing angle to the sun (the big studio light in the sky) causes the lunar phases. (Top: NASA photos / Bottom: Bob King)

When that angle expands to 90°, exactly half the moon basks in sunlight, and it stands due south around sunset. You can actually see this angle with your own eyes. Face the half-moon, then extend your left arm and point at it. At the same time, point your right arm toward the setting sun. Now look at the angle between your arms, and it should be dang close to 90°, also called a right angle. We call the half-moon the first quarter moon because it's moved a quarter of the way around its orbit since new moon.

By the way, the half of the moon you can't see at this time is still there but under cover of darkness. Be patient. In the coming nights, as the moon waxes (increases its phase) from half to gibbous, the sun will shine here soon enough. Gibbous comes from the Latin word gibbus, meaning "hump."

The sun illuminates the entire lunar nearside only at full moon when it's directly opposite the moon in the sky. In terms of angles, the sun and moon are then 180° or half a sky apart. Sunlight streams past the Earth's day-lit hemisphere and shines directly on the moon as seen by an observer on the Earth's nightside.

Just for fun, try to picture the lunar farside during the next full moon. As you might imagine, when we see a full moon, it's night or new moon on the farside because that hemisphere is completely hidden from the sun. And when it's new moon for us — you guessed it — it's full moon on the farside.

In two weeks, we'll revisit the moon again and explore its waning phases from full to new. By the time we're done you'll have a new cycle for measuring time and change in your life.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.