Time: 6:00 AM
Brief
Late last month, I showed the stick figure of Scorpious, the Scorpion. Its brightest "heart" star Antares is a red giant, and usually visible under clear conditions. Since the waning gibbous Moon will be near it for the next few mornings, we can watch to see our satellite pass north of it. The image shows the Moon at about the same altitude as Antares, which as seen from our mid-northern latitude, is not too high above the horizon. However, even more than two hours after transit, they don't sink that much, so still high enough to see over most obstruction of some hills.
Detailed
The Moon's orbit follows the ecliptic plane as seen from Earth, with an inclination of about 5 1/2 degrees. As shown, the arrowhead marks the ascending node, for when the Moon moves north of the ecliptic; 180º away, is the descending node. What is most interesting about these nodes is that they make a full cycle of precession around the ecliptic about every 18.6 years. As it does, the most southern and northern parts of the orbit can be imagined to pass by many naked eye stars, meaning that the Moon will cover, or occult them briefly as it moves about one diameter an hour, west to east. Why do I bring this up? Antares is one of those stars! With an ecliptic latitude of 4.6º, Antares falls within the range of the Moon's inclination. Two years ago, in early 2010, was the last time that the Moon moved to Antares' ecliptic latitude, and the next time will be late 2023 through early 2024.
How is it determined when the Moon covers a star? A star that falls within the 11º range (5 1/2 multiplied by 2, north and south of the ecliptic) encounters two brief occultation periods that last for as long as 1-11 "Moon-months"; properly termed as the synodic period. Exactly how long depends on how close the Moon is to that 5 1/2º mark, where the orbit's "slope" changes the least. Look to the left of the image, where an example of this shows. Also, the closer that a star is to the extremes, the longer the Moon's orbit stay nears it, and therefore the Moon can occult it. Any star which is as far as an orbit extreme reaches only gets that one pass every 18.6 years, while those which are just outside the extremes by about 10-15 arc-minutes may only become grazed by the Moon one time during the long cycle. Depending on where the Sun is in the sky, either a full, gibbous, quarter or crescent Moon occults the stars, waxing or waning. It is best when crescents do, provided that they are not too close to new phase, and far enough away from the Sun so glare doesn't get in the way. Cresent Moons (1) don't cause as much glare in the sky being less bright in apparent magnitude; (2) we can see the star either appearing or disappearing from the dark limb of the Moon. What is just as cool, is when the Moon occults a planet. This can be more rare, since such means that the planet needs to be in the right part of the sky, and have nearly the same ecliptic latitude as the Moon.
I am including a photo series from a few years ago, when the waning crescent Moon was finishing occulting Venus in the morning sky. The planet is seen coming from behind the Moon's dark limb, during a time span of about three minutes. The photos were 4-times optical zoom-in with no telescope, although cropped to make both the Moon and Venus look a bit bigger. They were taken during dawn April 22nd, 2009, with the exposure later lowered to reduce the "noise" in the background.
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