The next full moon is only two days away (August 22nd at 6:02am MDT). The phase we see tonight is called the ‘waxing gibbous’ phase since it occurs after the first quarter phase and is moving toward full.
During the waxing gibbous phase, much of the Moon’s face pointing toward Earth is in lunar daylight. With the Sun high in the lunar sky, there’s not a lot of contrast between light and shadow, so what we see at this time are largely patterns of different shades… except on one part of the Moon. This is where my favorite lunar feature, the lunar terminator, comes into play.
The lunar terminator is the area of the Moon that separates day from night. During the waxing gibbous phase, the terminator marks the portion of the Moon that is experiencing sunrise (sunrise at a given location on the Moon occurs about every 27 Earth-days. This is because the Moon rotates once on its axis every 27.3 Earth-days – much slower than Earth’s rotational period of 24 hours).
From our vantage point on Earth, the lunar terminator slowly moves across the face of the moon from right to left. Shortly after ‘new moon’ (when the Moon is between the Earth and Sun and is not visible to us), we see a thin crescent moon in the western sky, shortly after sunset. The slim crescent is the portion of the Moon that we can see that’s in daylight (during the crescent phase, most of lunar daylight occurs on the far side of the Moon).
As the Moon moves in its orbit, the crescent gets wider, eventually becoming a first quarter moon. At first quarter, the right half of the Moon’s face is in daylight, the left half is in lunar night. Each night on Earth, the terminator (the ‘line’ between day and night), moves slightly to the left, eventually disappearing at full moon. After full moon, the sunset terminator appears on the right side of the Moon and marches toward the left. At this time, the Moon is in the waning gibbous phase, moving toward last quarter, the waning crescent phase, and then back to new moon.
Below are four photos of the waxing gibbous Moon, taken on consecutive nights in May 2020. If you look at each photo, the area of the Moon along sunrise terminator shows great sunlight and shadow contrast because the sunlight, as seen from the Moon’s surface, is low. Lunar features that are normally unremarkable stand out in vivid detail during this transition from night to day.
Photo 1. Waxing gibbous moon, taken May 2, 2020.
In Photo 1 above, taken on May 2, 2020, you’ll notice at the terminator, near the left edge of the photo, an arc of a crater wall that contains a flat area of lava (the crater wall looks like the letter C). The flat area is known as ‘Sinus Iridum’ or ‘Bay of Rainbows.’ Much of the crater wall surrounding Sinus Iridum is on the night side of the sunrise terminator, but it’s high enough to catch the light of the rising Sun, even though the lava plain below the wall is mostly still in night’s shadow.
Photo 2: Waxing gibbous moon, taken May 3, 2020.
On May 3, 2020 (Photo 2, above), Sinus Iridum and its surrounding crater wall are in full sunlight and new features have moved into view. For example, a large, round lava basin known as ‘Mare Humorum’ (the ‘Sea of Moisture’) has appeared at about the 6:30 position on the Moon. A younger crater named Gassendi, with a central peak, can be seen at the upper left of Mare Humorum. These features were in lunar night when Photo 1 was taken, but can now be seen in detail, given the interplay of light and shadow near the terminator.
Photo 3: Waxing gibbous moon, taken May 4, 2020.
On May 4, 2020 (Photo 3, above), the crater Gassendi is less obvious since the Sun is higher in its sky and the sunrise has brought other features, including the crater ‘Aristarchus’ into view. Aristarchus is the small, bright white crater, just inside the terminator, at the 8:00 position. Aristarchus is so white that it can be seen with the naked eye.
Photo 4: Waxing gibbous moon, taken May 5, 2020.
Finally, on May 5, 2020 (Photo 4, above), the sunrise terminator has moved far enough to bring the crater Grimaldi into view. Grimaldi is the large, dark-floored crater at the 7:00 position. It looks like the bottom third of a ‘snowman’ of craters that lies just inside the terminator.
Also, the large crater Clavius is now visible at the 5:00 position (it’s also about 5:00 from the bright, rayed crater Tycho). Clavius is the second largest crater on the visible side of the Moon. Because of the light and shadow contrast along the sunrise terminator, you can see at least one small crater on the floor of Clavius.
If you pick any feature in Photo 1 and find it in the following photos, you’ll notice that as the feature moves away from the terminator, it becomes less distinct as the contrasting light and shadows lessen. But, every night on Earth, the sunrise and sunset lunar terminators, with their low sunlight and deep shadows, bring new features into view, in sharp relief. A few moments observing along the terminator with binoculars or a telescope is a highly rewarding experience. Even if you do this regularly, you’re bound to find something you hadn’t noticed before.