Cool Stuff To See On The Moon Tonight With Binoculars

The waxing gibbous moon will be a couple fists to the left (east) of Mars and Saturn this evening just above the "Teapot" of Sagittarius. Stellarium
The waxing gibbous moon will be a couple fists to the left (east) of Mars and Saturn this evening just above the “Teapot” of Sagittarius. Stellarium

We’ll have an awesome moon standing up in the south at twilight this evening. It’s in waxing gibbous phase between half and full and spending the night in Sagittarius the archer. If you’ve got a few minutes and pair of binoculars, some really nice craters and seas will be in view. Even 7x binoculars will show the larger craters along the lunar terminator. This is the line dividing lunar day from night. Tonight’s terminator defines the curved edge (left side) of the moon and represents the advancing line of lunar sunrise.

As the moon’s phase waxes or increases, the terminator moves to the left (east), exposing more and more of the shiny disk until we see the complete circle at full moon. After full moon, the terminator becomes the advancing line of lunar sunset and moves to the right (west). Along and near the terminator, craters and mountains cast shadows in the sun’s slanting light, revealing crisp outlines and other features in detail. This is where you’ll want to focus your gaze as you examine the moon with binoculars.

Use this map from the Virtual Moon Atlas to help you identify our featured seas and craters tonight. Credit: Patrick Chevalley,
Use this map from the Virtual Moon Atlas to help you identify our featured seas and craters tonight. Notice how much richer in craters the moon’s southern hemisphere is. Credit: Patrick Chevalley and Christian Legrande

Toward the bottom or southern portion of the moon, you’ll see lots of craters but one will stand out as larger than the rest — Clavius. Clavius, named for Jesuit priest Christopher Clavius, a 16th-century German mathematician and astronomer, is 140 miles (225 km) in diameter and 2.2 miles (3.5 km) deep. At freeway speeds, it would take about 2½ hours to drive from one end of the crater to the other.

Clavius has lots of company. The southern half of the moon is jammed with craters but the northern half not so much. Why? Happenstance. Larger asteroid-sized impacts excavated huge basins across the central and northern parts of the moon which later filled with dark-hued lavas, erasing much of the older lunar crust. We see these giant impacts as the dark spots, better known as the lunar “seas”, that make up the face of the man-in-the-moon.

A trio of lunar craters — Copernicus (right), Kepler (center) and Aristarchus (top left) — with their rays of secondary craters created when material shot up by impact fell back to the surface peppering the area with secondary craters. Credit: Jim Misti
A trio of lunar craters — Copernicus (center right), Kepler (center left) and Aristarchus (upper left) — display gorgeous rays of secondary craters created when material shot up by impact fell back to the surface, peppering the area with secondary craters. Credit: Jim Misti

The southern section remains as the much-bombarded, original crust of the moon aged 4.5 billion years. Astronomers call these regions, which appear white to the naked eye, the lunar highlands. The seas by contrast are about a billion years younger.

Working our way up from Clavius we encounter Mare (MAH-ray) Nubium (Sea of Clouds) and then the magnificent crater Copernicus, astride the terminator. Copernicus, named for the Polish astronomer who shattered the old Earth-centered universe paradigm in 1543, is 58 miles (93 km) across and 2.4 miles (3.8 km) deep.

You’ll notice a pale white coloration to the right (west) of Copernicus — these are rays and represent material expelled by the impact that excavated the crater. All that spinning, tumbling rock fell back to the surface moments later and dug out thousands of smaller craters in a ray-like pattern. Splat!

The lunar topsoil or regolith is dark from bombardment by the solar wind, cosmic rays and micrometeorites. These cause iron in lunar soil to melt and vaporize and produce a dark coating around the surrounding minerals. When all those hunks of lunar crust came raining down, they broke through the surface and exposed lighter, less weathered dust and rock, the reason the rays are so much brighter than their surroundings.

Copernicus (left) is much deeper than Plato. You can see this even in binoculars when the two are near the terminator as they will be tonight. Credit: Damian Peach
Copernicus (left) is much deeper than Plato, the floor of which flooded with lava long ago. You can even see the difference in binoculars when the two are near the terminator as they will be tonight. Credit: Damian Peach

Keep going up from Copernicus and you’ll soon arrive at the moon’s outdoor swimming pool, Plato. Of course, there’s no liquid water on the lunar surface, but the crater’s oval shape and dark color resemble a swimming pool or lake seen from above. Plato is 68 miles (109 km) across and just 0.6 miles (1 km) deep. Why so much shallower than our other featured craters? Similar to what occurred to create the lunar seas, lava flooded up from below to partially fill Plato. That also accounts for its dark color is caused by a high iron content in the lava. Exactly like the lunar seas.

There’s much we can learn about the moon’s evolution with just a pair of binoculars. I encourage you to follow our satellite through its phases and identify additional craters and seas. If you don’t have a map, no problem. Just download a copy of the Virtual Lunar Atlas by Christian Legrand and Patrick Chevalley and you can see what to look for any night of the year.

8 Responses

  1. caralex

    Bob, how far away is the horizon, if you were standing on the moon? If you were in the centre of Plato, would you see its walls? I imagine you would, in Copernicus, as its much deeper.

    1. astrobob

      I love your question, Carol, and I think you’ll be shocked at the answer. For a person my height — 5 ft. 7′ — the horizon would be just 1.5 miles away. While that’s surprising, using the same height, the horizon on Earth isn’t a whole lot further: 2.9 miles. Again, that’s assuming you’re on flat surface looking out over a vast flat surface. Naturally, if there are mountains, you’ll see much further, but I don’t know the formula for calculating that. I’ve read in a couple places that you wouldn’t be able to see the rim of Copernicus from the center of the crater 29 miles away.

      1. caralex

        Thanks, Bob. There are formulas for calculating the curvature of earth, which have become more noticeable on various sites, given the flat earth nuttery that has taken over the webs of late. I don’t know one for the moon, though. Amazing to think you could stand in these vast craters and not see the walls, even at the height of Copernicus’ rim!

  2. RC

    Bob & Caralex,
    If you were standing on a sphere, you could form a triangle between your eye, any point on the horizon, and the center of the sphere. The line from your eye to the horizon would be tangential to the sphere, so the corner at the horizon would be a 90 degree corner on the triangle. The side of the triangle from the horizon to the center would be equal to the radius of the sphere, and the side between the center to your eye would equal the radius plus your height (this is the hypotenuse). Using the Pythagorean theorem, we can solve for the distance of the third side (your eye to the horizon).

    Earth’s diameter is 7917.5 mi., so the radius is 3958.75 mi. Knowing this, we have two sides of our triangle A=3958.75 mi, C=3958.75 mi + 5ft + 7″. Using A^2+B^2=C^2, we can solve for B, this is the distance to the horizon. B=2.894 miles. I did a quick google search to confirm this and found that a 6ft tall person can see 2.9 miles to the horizon, so this seems right so far.

    The Moon has a diameter of 2159 miles. Plugging the radius of 1079.5 miles into our equation, we’ll find that you can see 1.511 miles to the horizon on the moon. Because the moon is smaller, there is more curve, and you can’t see as far!

    The crater Copernicus is 58 miles across, and 2.8 miles deep. At the center, it’s 29 miles to the rim. So, if the rim of the crater can see more than 29 miles it will be able to see the center of the crater (and therefore the center of the crater will be able to see the rim). Going back to our equation A=1079.5mi, C=1079.5 mi + 2.8 mi. And when we solve for B, we find that B=77.8 miles. So, yes, you would see the rim of the crater from the center. In fact, from the rim, you could see the entire crater!

    I’m sure there are many ways to nit-pick this (the moon isn’t exactly a sphere!, etc), but this should be a good rough estimate which clearly shows that you’ll be able to see the rim from the center.

    Oddly enough, if you were in the center of the crater Plato, you’d have a difficult time seeing the rim. This is because it’s wider at 68 miles across, and only .6 miles deep so the rim isn’t so high up in the air! If you were on the rim of Plato, you would NOT be high enough to see the other side!

    I hope this makes sense. Some visual aids sure would have helped, but I gotta work with what I have!! 🙂

    1. astrobob

      Hi RC,
      This is excellent to know. I scoured several sources (some good ones) and each said that the rim of Copernicus would be invisible from the center. How nice to see your equation! I was looking for this formula the whole afternoon. Of course it’s the Pythagorean Theorem. Thank you so much!

    2. caralex

      Thanks, RC – that was an excellent explanation. i understood what you were saying all right, having read multiple explanations over the past year or so of how to calculate Earth’s curve, for the flat earth denialists.

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