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As the moon lights up the sky this evening, everyone will see the moon fairly close to Regulus, the brightest star in the constellation Leo. The moon is not flat, but tonight’s first quarter moon does appear to have a “flat” edge to it.
The apparently flat edge of the moon at the quarter phase is called the “terminator.” No, that is not a reference to a Schwarzenegger sci-fi movie. Instead, it refers to the fact that sunlight “terminates” or ends at this line.
In other words, it is the sunrise line on the moon. In the lit portion, it is daytime, and on the other side of the line, it is night. In reality, it is not correct to refer to the “dark side of the moon” as any permanent place, because with a constantly moving terminator, any location on the moon is in the dark half of the time, and in the daylight half of the time.
Sometimes astronomers refer to tonight’s moon phase as the “letter D” moon because of its apparent shape. It appears with a similarly “flat” edge at last quarter phase, but fewer people notice the backwards “D” because of the timing.
The moon’s phases are a result of the moon’s changing position relative to the sun and Earth. Check out this really great Flash demonstration: How to simulate lunar phases.
Before sunrise tomorrow for binoculars:
With an unobstructed eastern horizon and binoculars, you might be able to spot the Venus/Jupiter conjunction before sunrise tomorrow (May 11). Look for these two worlds very low in the east about 60 to 30 minutes before sunrise. Venus, the brighter of the two, ranks as the third brightest celestial body to light the heavens, after the sun and moon. Jupiter shines as the fourth brightest. This conjunction will be much easier to see from the southern hemisphere, because these planets rise sooner before the sun in that part of the world.