Did you know that lunar eclipses occur only at full moons and are visible anywhere the moon is above the horizon at the time? And are you aware that the lunar eclipse Thursday, Nov. 18, is the longest partial lunar eclipse of the century? Put the coffee on: It’s gonna be a long night, and it will be worth it.

It’s all geometry

An eclipse begins with a straight line. Remember that a line is made up of a set of points that extend in opposite directions. In an eclipse, those set points are the sun, Earth and moon. As the Earth passes between the sun and moon, the planet casts its shadow on the moon. Since the moon’s orbit is tipped around 5 degrees off-plane in relation to the Earth and sun, that straight line usually isn’t entirely straight. A lunar eclipse is the result of the moon crossing that plane of the Earth and sun at “nodes” at the same time as a full moon.

Types of eclipses

There are three basic types of lunar eclipses:

• Penumbral eclipse: When the moon passes through the Earth’s penumbral shadow
• Partial eclipse: When the moon only partially enters the Earth’s umbral shadow
• Total eclipse: When the entire moon passes through the Earth’s umbral shadow

What to look for

Late on Thursday, the moon enters the outer region of the Earth’s shadow called the penumbra. At this point inside the Earth’s shadow, the Earth appears to cover part of the moon’s disk, but not all of it. The moon will be noticeably dimmer.

When the moon enters the umbra nearing totality early Friday, Nov. 19, the color it takes on can vary. Astronomers use the 0-4 Danjon Scale to rate the moon’s look, ranging from “nearly invisible” to “copper.” This near-total lunar eclipse is anticipated as a 3 on the scale, described as a brick-red moon that might have a yellow edge.

Regarding the duration of the eclipse, the average lunar eclipse lasts about 90 minutes. This eclipse runs for about 3 hours and 30 minutes. That’s the longest duration of a partial lunar eclipse in over 580 years. OK, it’s gonna take a full pot of coffee!

When to watch

• 11:03 p.m. Moon begins to enter the Earth’s penumbra
• 2:04 a.m. Moon at greatest eclipse (center of umbra)
• 5:04 a.m. Moon departs the Earth’s penumbra

Look at the full moon at around midnight, and you will see a growing red hue if skies are clear. Keen observers will recognize the subtle color difference in the shadow, and some will not. The best time to view the umbra shadow is mid-eclipse at 2:04 a.m. Then, the moon’s color will be a rusty brick red.

Since the eclipse is mostly in the early morning, the bright autumn and winter constellations will ride high in the southern and western skies framing the event. About three-fingers width to the north of the moon at mid-eclipse is the constellation Pleiades. The “mini dipper” will be an amazing contrast to the red moon with its cluster of light-blue primary stars.

Finally, look at the brighter background stars right next to the moon, and watch them get eclipsed by the moon, too! This is called an occultation.

There’s so much to look at Thursday night and Friday morning that the time will fly by. Set the alarm clock, put out your warm clothes, grab the binoculars, get the coffee ready and enjoy some cosmic geometry! Of course, you can go back inside if you get cold or need to take a quick nap. Heck, it’s the longest lunar eclipse of the century. You’ve got time, but you can’t afford to miss it.

Happy near-total lunar eclipse and clear skies to you!

“Thinking Outside the Classroom” publishes periodically in the Summit Daily News. Mark Laurin, known as “Astro Mark,” is an adjunct instructor at Keystone Science School. Contact him at astromarka@gmail.com.

“Astro Mark” publishes periodically in the Summit Daily News. Mark Laurin, known as “Astro Mark,” is an adjunct instructor at Keystone Science School. Contact him at astromarka@gmail.com.