Crescent Moon Tilt
Why is the crescent moon at sunset sometimes tilted like a backwards "C", and sometimes flat on its back like a "U"?
Introduction: This is a tough question, and one that even the experts make mistakes on. First, the pointed parts of the crescent are sometimes referred to as the moon's "horns". Because the moon is just a big rock out in space, it doesn't glow like a star. So we only see the part that is lit up by the sun. Therefore, the crescent moon's "horns" will always be pointed away from the sun.
Short Answer: The cause of the crescent moon's tilt is similar to the cause of the seasons, but more complicated. The short answer is that it's caused by the fact that the Earth is tilted 23.5 degrees, and that the Earth orbits around the sun. In Madison, Wisconsin (USA), and at similar latitudes, this causes the waxing crescent moon to look like a backwards "C" in September, and a "U" in March. [See the images at the top of the page.] This difference has been noted throughout history, and has become the subject of folklore and superstitions, such as referring to these as a "wet moon" and a "dry moon", with misguided weather and climate connections. Don't worry too much about which is "wet" and which is "dry", because you can find references each way.
Okay. Hold onto your hats for the long explanation.
- The moon doesn't shine by its own light. We only see the part that the sun is shining on.
- A crescent moon is difficult to see in the daytime, so we usually notice it either above the sunset or sunrise.
- The moon orbits around the Earth in about a month, and it orbits the Earth in almost the same plane in which the Earth orbits around the sun.
- The Earth rotates once every 24 hours from west to east which makes everything out in space look like it's going from east to west in our sky. Sunset is when the Earth has rotated to the point where we are on the interface between the side of the Earth that the sun is shining on, and the side of the Earth that is dark. Check out the following animations to watch the crescent moon set in Madison, Wisconsin in spring compared to fall:
1. Spring Crescent Setting Animation
2. Fall Crescent Setting Animation
- The Earth is tilted 23.5 degrees from vertical in relation to the Earth's orbit around the sun. That tilt stays the same as the Earth orbits the sun, so the North Pole of the Earth is always pointed out in the same direction in space.
- The Earth is so large that it seems flat when we are standing on it, and our horizon (the ground extended in all directions) seems flat. So, even if we are on what looks like the "side" of the Earth from out in space, we still stand perpendicular to the ground.
- Our view of things out in space depends on where we are on the Earth, and where the Earth is in its orbit around the sun.
Long Answer: They say that a picture is worth a thousand words, and this is certainly true in astronomy. If you understand all of the background information above, then you are ready to wrestle with the following pictures to wrap your brain around this difficult concept. To explore the following diagrams/simulations, you'll need to know the following information:
- The tilt of the crescent moon varies based on your latitude and the time of the year. The images below were created for Madison, Wisconsin, but any location in the United States and similar latitudes will see the same thing. But at the equator, the crescent moon looks like a "U" in both December and June. And in the mid-latitudes of the southern hemisphere, the crescent moon most oriented like a "U" would be in September.
- For each of the images "as seen from Madison", it's set for a waxing crescent moon 3 days past New Moon, and the time is sunset.
- The green line in the "as seen from Madison" images shows the plane of the Earth's orbit around the sun. The moon's orbit is very close to the plane of the Earth's orbit. It's only inclined 5 degrees. So, to simplify the explanation, and to make it fit very well for every year, you can just take the tilt of the moon's orbit out of the equation. Therefore, for any given year, the moon could be on either side of the line, or on the line.
- The green lines in the "as seen from space" images show the moon's orbit and the Earth's orbit.
- The "as seen from space" images require some imagination in order to place yourself into that situation. First, explore the following explanation pictures before you dive into the ones below:
[Credit: all of these images were created using Starry Night Pro.]
Explanation picture for March
Explanation picture for September
- Click on each image below to view the larger image.
March: as seen from space.
March: as seen from Madison.
June: as seen from space.
June: as seen from Madison.
September: as seen from space.
September: as seen from Madison.
December: as seen from space.
December: as seen from Madison.
|[Credit: these images were created using Starry Night Pro.]|