Tonight – on the night of July 24-25, 2018 – the moon shines closest for this month to the planet Saturn. 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. If you keep watching the moon at the same time over the next several several nights, you’ll find the moon in a more easterly location than it was the night before. That’s always the case, because the moon moves continually toward the east in its orbit around Earth. In the days ahead, watch the moon approach the red planet Mars, the fourth-brightest brightest celestial body to light up the sky, after sun, moon and Venus.
Tonight’s 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. The waxing moon will finally turn full on July 27, which – quite by coincidence – is the same date that Mars reaches opposition. What’s more, this upcoming full moon will sweep right through the Earth’s dark shadow to stage a total eclipse of the moon.
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. Sometimes, it’s better to view the moon in a daytime sky because the moon’s glare canbe overwhelming in a nighttime sky.
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 24-25, 2018 – 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.