Event Date: February 2nd
Time: 12:00 AM
Brief
About two weeks ago, I showed the first-quarter Moon high in the sky, far north of the Sun. Now, as the Moon wanes to last (3rd) quarter, it has moved much further south of where it was near full, about a week ago. It is not as far south as the Sun is now, yet with every rise since turning full, the gaps have been--on average--slightly over an hour apart. This is much different than Moonrises of the waning gibbous Moon 5-6 months ago, when I emphasized how it rose about half an hour later each evening. Why the difference?? Simple: then it was moving north quickly, and this time, south! Look at the image below, at where it is in the sky: about where the Sun is in late October.
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click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.
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Detailed
When looking above, we see how the ecliptic's angle with the horizon is large. Therefore, it means that west of the Moon along its orbit, were declinations more north than it is seen here. The further south it moves, the longer we have to wait for it to rise, compared to the day/night before. However this time, as a bit of a "twist", the Moon's orbit is bringing it closer to ascending node and therefore, the rise-gaps have remained about the same over the last 6 dates; only increasing slightly. As the Moon continues to wane after 3rd quarter, the combination of the the ecliptic becoming more "parallel" with the equator (at least seen that way with the software or if imagined from Earth-viewing), the gaps between rises will shrink rapidly. Also, the orbit of the Moon bringing it north of the ecliptic will contribute to this. The last few days before new Moon, we will see the old, waning crescent rising at gaps very similar to full and waning gibbous Moons in the late summer/early fall. Think six months ahead, or six months back, when comparing Moon-rise times for phases about half a cycle opposite each other.
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