Tonight – July 25, 2015 – the moon shines closest for this month to the planet Saturn. If you’ve been watching over several nights, you know that this moon is in a more easterly location than it was on July 24. Of course it is! The moon moves continually toward the east in its orbit around Earth.
For illustrative purposes, the moon’s size is exaggerated on our sky chart, so the moon won’t appear as close to Saturn in the actual sky as it does on our sky chart. The exact position of the twosome depends on where you live and at what time you’re looking.
At its closest, the moon comes to within two degrees (four moon-diameters) of Saturn, plenty close enough for the duo to fit within a single binocular field.
The July 25 moon is a waxing gibbous moon. That means it’s more than half-illuminated but less than fully illuminated in sunshine, as seen from Earth. In North America and much of the world, the moon reached its half-lit first quarter phase on the night of July 23.
The waxing moon will finally reach full phase on July 31, to feature a Blue Moon – the second of two July 2015 full moons!
We got this comment from an online friend:
I was driving down a lonely stretch of dark Georgia highway the other night heading west. This gave me plenty of hours and miles to contemplate the moon . . . Is there a formal name for the border of lunar night and day?
There is indeed a name for this demarcation between light and dark on the moon. When you look at the moon in any phase other than full, you are in fact seeing portions of both the day side and night side of the moon. The name for this border between lunar night and day is the terminator line.
If you’re using a telescope or binoculars, and want to observe features on the moon’s surface, one trick is to look along the terminator line. There, shadows are causing lunar craters and mountains to stand out in stark relief in contrast to the surrounding plains. This is, after all, the line of lunar sunrise, and in some ways it’s similar to the long shadows we see on Earth at sunrise or sunset.
If you were standing at the location of that line on the moon’s surface, you’d be standing at the edge of day, or night. A similar line on Earth’s surface passes over you each day at sunset and sunrise. But there is one major difference. On the moon, there’s no dusk or dawn, because the moon doesn’t have any air to disperse sunlight, and to create the twilight that we see on Earth.
Bottom line: On this night – July 25, 2015 – look for the waxing gibbous moon near the planet Saturn. The line between lunar day and night is called terminator line, and it’s the best place to look on the moon with a telescope or binoculars.