Crescent Moon and near-the-cusps Venus: When last? When next?

Every so many years Venus is close enough to the cusps of a crescent moon to have a reasonably close appearance to the crescent and star motifs found in Islamic iconography. When was the last time, and when will be the next time? (If it makes a difference, Eastern US.)

I definitely remember one in the early to mid 1990s, but there might have been others since then.

Interesting question. The location definitely makes a difference, as the moon sweeps across the sky at about half a degree per hour, so the phenomenon will not last long.

We can start by calculating when the moon passes within, say, half a degree of Venus, with the sun below the horizon. A rough calculation (at least one which you should verify) gives a conjunction with a thin crescent moon before sunrise on June 19th, 2020 (starting around 8:20 Universal time), and a really awesome encounter on 13 September 2031 around 8-10 Universal time.

Going back, it happened at moonrise on July 12, 1996.

So was the juxtaposition a few days ago (the 18th I think) close enough? That had Venus about three degrees to one side of the moon and sure was pretty. DPRK posits a separation of one half degree and that is about the distance between the cusps, so pretty darn close.

I just did an image search and for a lot of those depictions the answer is “never”. The majority (at least of the first my search on “Islamic moon and star” in Google imagery) showed the star-like object inside of the cusps of the crescent and thus an image which never happens because the star-like object would have to be closer to the observer than the moon. FYI…

For the best case, you want times when the Moon passes directly in front of Venus from a particular viewing spot on Earth. This is called a lunar occultation of of Venus, and there are a number of online calculators that let you find future and past dates and locations. See here or here, for example.

From these we see an occulatation of Venus visible from the eastern US on July 31 of 2019, and another in March of 2024, and another in June of 2026. It looks like an occultation of Venus is visible from somewhere on Earth roughly twice a year on average. The NAOJ site gives you animations of the paths.

The greatest possible angular separation of the Sun and Venus as seen from Earth is 47 degrees, so interestingly any time Venus and the Moon are near each other in the sky, the Moon will be a crescent if it’s visible at all (not new). Another way of saying that is that anytime the moon is more than half illuminated it’s in the wrong part of the sky to be near Venus.

The best solar eclipse I’ve seen was in 1998, from the island of Guadeloupe. Totally clear skies, and Venus and Jupiter were very close to the moon, seen during totality.

Wait… is there a discernible change in the relative position of the moon with respect to the planets within a single night?

I thought more about this, and back-of-the-enveloped it, and I think I underestimated it by quite a bit. Put the moon at 47 degrees to the Sun, cos 47 is 0.68, (1-cos 47)X0.5, math math math, it looks like the moon would have to be a slim crescent of only 16% illuminated at occultation at best, when Venus was at its greatest possible angular separation from the Sun.

For USCDiver, yes, for the 2019 occultation the Moon will sweep across Venus in about an hour.

…and not only within a single night, possibly within the course of a leisurely dinner out. I was going in to dinner with friends one evening when the moon chanced to be close to a couple of bright stars. Being an astronomy geek I pointed out to my companions to note carefully the positions of the moon and nearby stars. Sure enough, when we came back outside after a couple of hours the moon had shifted about two moon diameters from its original position…

Will the OP actually be able to see something of the occultation on July 31 2019 in the middle of the day? Also 2024, etc


Having just recently watched the shadow of the moon race across the county in the course of an afternoon this should have been patently obvious to me!

If I may piggy back a related question then: How much does the phase of the moon change in one night?

What determines how much of the moon’s disc is illuminated is the elongation between the Sun and the Earth, measured from the center of the Moon. (Or you can just use the supplement of the geocentric elongation of the moon from the sun, which will be pretty close since we are relatively far from the sun.) If this angle is ι, then the illuminated fraction is (1 + cos ι) / 2. For example, when ι = 0° the Moon is full, and when ι = 180° the moon is dark.

The problem is that the Moon’s motion is extremely complicated. But if you want a really, really rough rule of thumb, the most important effect will be the average synodic month of approximately 29.53 days. In other words, to a first approximation, the phase will vary sinusoidally with this period. The phase is changing most slowly when the moon is new or full, and most rapidly at the quarters.

For example, the moon’s First Quarter was Sunday at 21:45. The moon rose in Chicago at 16:42 (GMT) and set at 7:35, during which time the phase increased from 47.7% to 54.8%. On the other hand the Moon is full on April 30, and during the time it is up the phase only decreases from 99.8% to 99.6%.