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The lunar terminator is the dividing line marking the edge between day and night on the Moon. It is sometimes called the twilight zonebecause it marks where the sun either rises or sets, bringing the lunar landscape there into a state of twilight. Want to see lunar features more clearly with binoculars or small binoculars? Look along the terminator line. It is there that the contrast between light and shadow reveals the topography of the Moon in sharp relief. Mountains rise above it lunar regolith – dislodged dust, broken rocks and other materials covering most of the lunar surface – while craters drop from their rocky ledges.
Terminator line in motion
Of course, the Terminator isn’t fixed on the moon. It creeps across the surface of the Moon, with its change in position visible to those looking through binoculars in mere hours. A day on the Moon lasts 29.5 Earth days. The lunar terminator moves at slightly slower than 10 miles per hour (16 km/h) across the surface of the Moon. This pace seems almost glacial when you consider that for those on Earth’s equator, our world terminator is flashing across its surface at about 1,600 km/h.
If you set your treadmill to 10 miles per hour (16 km/h) to reflect the speed of the lunar terminator, you could jog (or rather run) and see if you can keep up with the lunar twilight wave. You would have to run at a pace that equates to about 6 minutes and 15 seconds mile. (FYI, if you could run that fast for 26.2 miles, you would be considered an elite marathon runner).
The Terminator and the phases of the moon
As seen from Earth, the lunar terminator marks the line of lunar sunsets or sunrises. As the Moon orbits the Earth to the east, sunrise terminator intersects our satellite. This creates the familiar phases we recognize as waxing crescent, first quarter, waxing gibbous, and finally full. During a full moon, we see no terminator at all because the side of the moon facing us is completely illuminated. After a full stage, sunset terminator the Moon goes through the phases in reverse. The phases run from waning gibbous, last quarter and waning crescent to new moon when no light illuminates the surface.
Experienced astronomers know that observing the Moon at its full phase is a washout. The moon is super bright, its surface is bright but dull, and your night vision is ruined by looking through binoculars at such a blazing light. On the other hand, the waxing and waning phases of the Moon provide a detailed study of the lunar surface one night at a time over the course of two weeks.
Read more: Here are 4 keys to understanding the moon phases.
A straight wall
One popular feature that sees the terminator in its “best light” is the monthly Straight Wall. Straight wall, also known as Rupes Rectais the fault line that lies within Mare Nubiumbasin in the southern part of the moon. The straight wall is one of those features on the Moon that can almost disappear, except when the terminator casts its conspicuous shadow on the eighth day after the new moon. If you caught a Rupes Recta and it was a piece of cake to find, try a smaller one Rima Birt running parallel nearby.
Lunar works best on terminator
Even the more obvious lunar features, such as large craters, benefit from being viewed when a terminator passes over them. For example, craters Copernicus and Eratosthenes on Apennine Mountains best to show their altitude in contrasting light.
Terminator line on other bodies
But wait! The moon is not the only object in the solar system that displays a terminator.
Any planet and satellite with an external light source such as the sun can display a terminator.
Inferior planetsthose inside Earth’s orbit show the terminator and phases more prominently. So, for example, we can see the phases of Venus and Mercury on Earth. And any Martians on Mars can see Earth’s phases and its terminator line.
Amateur astronomers know the phases of Venus because they are easily visible through binoculars or binoculars. Venus’ disk appears larger and crescent-like when it is closer to Earth. It looks smaller and like a rounded phase when it’s further away.
Mercury’s proximity to the Sun means that the solar wind has blown away its atmosphere. Therefore, the separation of the terminator line between day and night causes a sharp fluctuation in temperature. When the sun hits the day side of Mercury, surface temperatures climb to 800 degrees Fahrenheit (430 Celsius). But simply across the terminator boundary on the night sidetemperatures drop to -290 Fahrenheit (-180 Celsius).
Bottom line: The Moon’s terminator line divides day and night on the Moon. As we see it from Earth, it marks the line of moonsets or sunrises. Other objects, including Earth, have a termination line.
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