Star Points: April's total eclipse of the moon

The big news this month is the upcoming total lunar eclipse taking place from early April 14 to April 15 in the late hours past midnight. In other words, the eclipse takes place very early Tuesday morning as the moon glides through Earth's shadow, which is being projected into space by the sun.

The only instrument needed to observe any lunar eclipse is the unaided eye. Of course, optical boosts given by binoculars or a small telescope will enhance the experience all the greater.

A precise geometrical alignment is in effect as the moon passes through Earth's shadow and a total lunar eclipse takes place. The moon, Earth and sun need to be perfectly aligned, with the sun and moon in opposite directions from Earth. Thus, from Earth, a lunar eclipse may only be viewed at night when the moon is up and the sun is down. Because of this geometry, the moon phase is always full at the time of a lunar eclipse. To witness a lunar eclipse, an observer must be located on the side of Earth from which the moon is visible at the time.

The upcoming eclipse occurs in stages, between 12:52 a.m. and 6:39 a.m. April 15, with the most dramatic stages occurring between 1:58 a.m. and 5:33 a.m. The details for each stage are outlined below.

The "penumbral" eclipse stage begins at 12:52 a.m. as the moon first touches Earth's penumbral shadow. Earth has two distinct conical shadows shaped like very long dunce caps, with the cap wearer's head representing Earth. These shadows are concentric, or one is inside the other. The longer outer shadow is the penumbra and the shorter inner shadow is known as the umbra. Just picture a person wearing two long dunce caps, a smaller one inside a larger one.

An astronaut anywhere inside this outer penumbral shadow looking back toward Earth would see it blocking only part of the sun. The farther into the shadow they ingress, the less they see of the sun and likewise the shadow grows darker.

A person on Earth viewing the moon during the penumbral eclipse stage might notice nothing at first. But after a while they may perceive the brilliant full moon as being somewhat dimmed. I found this effect striking at last May's penumbral lunar eclipse. The unfiltered full moon was almost painfully bright when viewed in binoculars. But minutes later during the penumbral eclipse, not only could the moon be observed without discomfort, but in greater detail as well, due to its reduced glare.

The next stage is the "partial eclipse" beginning at 1:58 a.m. At that time, the moon begins slowly into Earth's umbral shadow. The moon will appear to have a deep darkening along its leading edge. The umbral shadow is the darker inner dunce cap in our hypothetical model.

As the partial stage progresses, the large circular dark umbral shadow appears to slowly devour the moon. Actually, what is happening is the moon is voluntarily plunging into the shadow all on its own in the normal course of its orbit around Earth.

If we were astronauts standing on the moon's surface within the umbral shadow's dark cone, we would see a magnificent sight. The sun is completely invisible because it is totally blocked by Earth. The surface of the Earth is dark because we are looking at its night side. As Sky & Telescope magazine's Alan McRobert describes it in his online blog, "you'd see Earth ringed with a thin, brilliant band of sunset- and sunrise-colored light."

This exact scene was predicted and produced using special effects in the 1929 classic German science fiction silent film "Frau im Mond," or "Woman in the Moon" (available on Netflix). German visionary and rocket scientist Hermann Oberth served as technical advisor for the film. Oberth's work, as well as the movie, were inspirations for the young Wernher von Braun. In later life, von Braun would lead a German rocket team in developing the deadly Aggregat-4 rocket (aka Vengeance Weapon 2, or V-2) during World War II. Von Braun's career would eventually culminate with the building of America's Apollo Saturn V rocket, which carried the first humans to land on the moon in 1969, some 40 years after the movie's release. For better or worse, events like these are a testament to the film's long-term historical impact through the tumultuous events during the decades of the 20th century.

As the film's cast members stand gaping at the view through their window on the way to the moon, they are transfixed at the beautiful sight of Earth brightly ringed by light of the eclipsed sun. The young Gustav, a lad who had stowed away aboard the rocket, solemnly remarks, "On our Earth the sun is just rising."

Gustav was partly correct. For that reddish ring is not only the light from every point on Earth where dawn's morning twilight is occurring, but it also includes the light from everywhere that dusk's evening twilight is happening at that instant in time. Some of this light is scattered moonward during the eclipse, resulting in the moon taking on colors ranging from copper red to dark dusky brown.

Notice how, as the moon darkens, stars that were not visible earlier due to the light of the full moon seem to be coming out even though it is already night. This effect is sometimes described as a "night within a night."

Finally, at 3:06 a.m., the "totality" stage begins. The moon is now completely inside the cone of the umbral shadow. It will remain completely engulfed by the umbra for some 79 minutes. The moon passes just south of the shadow's cental axis. Therefore, the moon's southern edge could remain lighter than the northern.

Once totality ends at 4:25 a.m., the stages occur in reverse, beginning with the partial eclipse. The shadow slowly recedes and the stars begin to fade again as the moon brightens. The partial eclipse ends at 5:33 a.m. as the moon exits the umbra. The penumbral eclipse then ends at 6:39 a.m. However, in the minutes leading up to the ultimate end of the eclipse, the sun rises and the moon sets.

You don't need to go anywhere special to view the eclipse. Just step out of your house and look up. If you want to view through a telescope, the Westminster Astronomical Society is planning to set up a telescope or two at the main Westminster branch of the Carroll County Public Library, weather permitting of course.

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