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    A short Moon day Saturday
    From: Frank Reed
    Date: 2024 Jun 21, 11:43 -0700

    The Full Moon reverses the Sun's seasonal behavior in the sky. If your live in a mid-northern latitude, you know that days in late December are short, and the Sun rides low in the sky at local noon. You also know that days in late June are long, and the Sun passes very high in the sky near local noon. The Full Moon is opposite the Sun in the sky, so it flips the rules. If there's a Full Moon in late December, it does what the Sun does in June: it's in the sky for many long hours and it reaches a very high altitude in the sky at meridian transit. By contrast a Full Moon in late June implies that the Moon will be in the sky for short hours, and its peak altitude will be low in the south.

    Tonight is a summer solstice Full Moon with a twist. Our expectations about the Moon's motion will be correct, but it's even more pronounced this year because of the tilt of the Moon's orbit. The "Moon day" --the time when the Moon is in the sky-- will be short and actually shorter than expected, and the maximum altitude will be very low in the southern sky.

    Picking 40°N and 60°W as a "standard location", I find, using Stellarium, that the (unrefracted) Moon rises at this location at 23:53:21 UT (21 June). By random luck for my choice of location, this is only about 18 seconds away from the time of Full Moon, as traditionally defined. The traditional definition calls it Full Moon when the Moon and Sun are separated in ecliptic longitude by 180° exactly. Equivalently, we can look in Stellarium at the "elongation in ecliptic longitude", which should be the same thing. Sure enough, that hits 180° exactly at 23:53:39.

    Normally, when it's exactly Full Moon, the Moon rises just as the Sun sets. That makes sense because the Moon and Sun are on opposite sides of the sky, opposite sides of the celestial sphere, at the moment of Full Moon. But tonight? At this selected location? The Moon is rising 26 minutes late --26 minutes after the Sun's center has hit the horizon. The "night" tonight (night, for now, I am defining as the period when the unrefracted center of the Sun has an altitude below 0°) is 9.16 hours long from the given location. That's nice and short because the exact solstice, maximum Sun declination, was yesterday. But tonight's "Moon day" (which I am defining as the time period when the unrefracted center of the Moon has an altitude above 0°) is not the same. It's considerably shorter. From this specific location, it's 8.54 hours long. They don't match. The "Moon day" today is shorter than the "night" by about 37 minutes. And the primary reason, of course, is because the Moon tonight is not, in fact, opposite the Sun in the sky. It has significant southerly ecliptic latitude, 5° below the ecliptic. The Moon's orbit is tilted down, and it's very close to that maximal tilt effect right now.

    Incidentally, the Moon's motion partially cancels this out. If we follow the spot on the celestial sphere where the Moon was located just as it rose, then we would find that this period from rise to set is shorter still, 8.27 hours, making it 53 minutes shorter than the "night". But since the Moon is moving "left" all night long at the usual rate of approximately half a degree per hour, its motion keeps it in the sky longer, cancelling out the extreme ecliptic latitude effect by 16 minutes.

    Frank Reed

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