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The Ever-Changing Face of the Moon – Twin Cities



We have a full moon this weekend, and as big and lovely as full moons they mess up good star observations with their whitewashing effects.

You can still see bright stars and some constellations and the bright planets Mars Jupiter and Saturn still show a show in the low southern sky. Mars has faded somewhat, but it is still the brightest in 15 years.

Mike Lynch

This week, I want to return to the basics of the moon: Why is the moon changing its shape? Why is it a crescent moon in one night, a crescent moon on another night and a full moon on another night? Why do not we see the moon on some evenings? Everything comes down to two things: the orbit of the moon around the earth and the sunlight reflected by the moon.

The moon does not produce its own light. It is all reflected sunlight as the moon turns around the earth. The angle of change between the moon, the earth and the background sun is responsible for the shape or phase that we see. The best way to explain the phases of the moon is to look at the different positions in its orbit.

New Moon: This is when the moon lies approximately in a line between the earth and the sun the sunlit side of the moon is completely turned away from the earth. The moon is invisible to us and its position in the sky is near the position of the sun rising at sunrise and sunset. Every now and then the moon lies exactly in a line between the earth and the sun, which leads to a solar eclipse.

Growing Crescent Moon: A few days after the new moon is the angle between The moon, the earth and the sun are opening a bit and we start to see a small splinter or crescent of the sunlit part of the moon , It rises shortly after sunrise and sets shortly after sunset, and we can see it after dusk in the western sky for a few hours. This is a great time to see a phenomenon called "Earthshine". That's when you see the crescent shape of the moon and also the rest of the moon faintly bathed in second hand sunlight reflected off the earth and the lunar surface. It is a nice spectacle!

First Quarter Moon: One week after the phase cycle, we have a first quarter moon. It's called "first quarter" because the moon goes through a quarter of its phase cycle. It does not mean you only see a quarter of the moon. You are actually seeing a crescent because half of the sunlit part of the moon faces the earth. The earth, the moon and the sun are at right angles to each other. The moon rises around noon and goes down at midnight. This is a wonderful time to see our moon neighbor with a telescope. Look specifically at the so-called "Terminator". This is the border between the dark part of the moon and the sunlit part. Along the terminator, the shadows are long and reveal features that are otherwise harder to see. You can even see the mountain peaks poking at the shadow on the dark side of the Terminator.

(Courtesy of Mike Lynch.)

Growing Gibbo Moon: Ten days after the new moon the angle between the moon, sun, and earth opens more than a right angle, and we see more of the sunlit half of the moon. Then the growing moon takes on an oval shape and starts a lot of light. The growing moon rises in the middle of the afternoon and leaves at 2 or 3 o'clock in the morning.

Full Moon: Fourteen days after the new moon is full moon. We are now in the middle of the moon's 29-day phase cycle. The moon is on the opposite side of the earth from the sun, and on the earth we can see the entire sunlit half of the moon. The full moon rises at sunset and sets at sunrise, and while moonlight is wonderful for love and romance, star-gazing is toast! This is also not a good time to explore the moon. Everything on the surface is in direct sunlight and there are no revealing shadows. Around the time of the full moon you can see the classic "Man in the Moon".

Sometimes the full moon gets in the reddish shadows of the earth and we see a lunar eclipse. This happens on average once or twice a year.

Decreasing Gibbon Moon: About 17 days after the new moon, we again see an oval moon, but this is the reflection of the growing gibbous. The angle between moon, sun and earth begins to close. The waning moon rises after sunset and sets after sunrise. That's when you see the moon in the western sky after the sun has already risen.

Last Quarter Moon: Twenty-one days after the new moon we have a last quarter moon. Again the moon, sun and earth are at right angles. We see the other side of the moon as we saw in the first quarter. The last quarter moon comes up around midnight and goes down around lunchtime. Just like the waning moon, after sunrise we see the last quarter moon in the western sky

Decreasing crescent moon: About 24 days after its new phase, the moon becomes a half moon again The angle between sun, moon and earth smaller and smaller. The waning crescent moon rises two to three hours before sunrise and sets in the early afternoon. It is so close to the sun that it becomes invisible shortly after sunrise. Before dawn, however, you still have a chance to see "Earthshine" or "second hand sunshine".


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