Science & Technology

All You Need to Know for Sunday’s Total Lunar Eclipse

January 16, 2019

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Aligning his camera on the same star for nine successive exposures, Sky & Telescope contributing photographer Akira Fujii captured this record of the Moon’s progress dead center through Earth’s shadow on July 16, 2000.
Sky & Telescope / Akira Fujii

North America, South America, and indeed much of the world are gloriously poised for the total lunar eclipse of January 20–21, when, for an hour, the Moon will turn a reddish hue. The eclipse will begin late on January 20th and continue into the small hours of the 21st.

Here are six things to bear in mind this coming weekend:

1. Who will see this eclipse?

If you’re in the Americas, Europe, or Africa the night of January 20–21, you’re in for a treat. This is the first total lunar eclipse visible across the contiguous U.S. since September 2015, and the next one visible anywhere won’t occur until May 2021. For viewers in the United States: Sunday night is the middle night of a three-day weekend — so the eclipse makes a good excuse to stay up late!

The table below indicates what to look for and when (Universal Times, or UT, are all for January 21st; local times are on the 20th if “p.m.” and the 21st if “a.m.”).

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North and South America will enjoy the full experience of the eclipse, from initial penumbral stage to final penumbral stage, during the evening of Sunday, January 20th, and into the night of January 21st. Viewers in the east of both continents will have to stay up into the early hours of the morning of January 21st to see all stages of the eclipse, while those in the west will be able to get to bed before midnight on the same evening.

Those on the islands of Hawai‘i will see a partially darkened Moon rise at sunset, as the eclipse will already be in progress. For most of Europe and Africa, the Moon will set either during totality or during the final penumbral phases, although westernmost Europe — including the British Isles — and northwestern Africa will be treated to the whole show.

This January’s eclipse will take almost 3½ hours from the beginning of the first partial phase at 10:34 p.m. EST (7:34 p.m. PST) to the end of the second partial phase at 1:51 a.m. EST (10:51 p.m. PST). The total phase of the eclipse will last 63 minutes, with its midpoint at 12:12 a.m. EST (9:12 p.m. PST).

2. What is a lunar eclipse?

A lunar eclipse happens when the Sun, Earth, and Moon line up in space. The Moon gradually glides into Earth’s central shadow, the umbra, until the entire lunar disk turns from white to an eerie dim orange or red.

Although the Moon is completely inside Earth’s shadow, it’s still dimly lit by sunlight that skims the edge of Earth and is refracted (bent) and scattered into the umbra by the atmosphere.

“That red light on the Moon during a lunar eclipse comes from all the sunrises and sunsets around the Earth at the time,” says Alan MacRobert of Sky & Telescope. “If you were an astronaut standing on the Moon and looking up, the whole picture would be clear. The Sun would be covered up by a dark Earth that was ringed all around with a thin, brilliant band of sunset- and sunrise-colored light, bright enough to dimly light the lunar landscape around you.”

3. What should you look for during this eclipse?

You need only your eyes to see the dramatic changes in color and brightness as the Moon slips into and out of Earth’s shadow. The Moon will take on an ever-darkening shading that morphs into what looks like a “bite” taken out of the Moon and culminating in the reddish hue of maximum eclipse; the steps then reverse as the Moon emerges from totality.

Binoculars will help you see the colors more vividly, while a telescope will reveal subtleties in those colors as well as the progress of Earth’s shadow on individual craters and other lunar features.

French astronomer André Danjon devised a five-point scale to describe the brightness and hue of a lunar eclipse at totality, ranging from 0, when the Moon is practically invisible at totality, to 4, when it’s at its brightest copper-red or orange.

The Moon will be in the constellation Cancer as the eclipse is unfolding — while you’re out watching the eclipse, look about three fingers’ width to the left of the Moon to spot the Beehive Cluster, one of the closest open star clusters to Earth. The stars Pollux and somewhat dimmer Castor, in Gemini, will be situated to the Moon’s upper right.

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This map shows locations worldwide from which the January 20–21 total lunar eclipse is visible, weather permitting. Because an eclipsed Moon is always full, the Moon sets (or rises) at almost the same time as the Sun rises (or sets) on the opposite horizon. For viewers in North and South America, the entire eclipse will be visible.
Sky & Telescope; source: Fred Espenak

4. What are the stages of a lunar eclipse?

  • Penumbral stage: The Moon’s leading edge enters the pale outer fringe of Earth’s shadow, the penumbra. From the Moon’s perspective, the Sun is only partially blocked during this stage; from Earth’s perspective, the Moon has a slight, dark shading that may be hard to detect.
  • Partial eclipse: The Moon’s leading edge enters the umbra, the cone of Earth’s shadow within which the Sun is completely hidden. The shading becomes more obvious as the Moon moves deeper into Earth’s shadow.
  • Total eclipse: The last rim of the Moon slips into the umbra and the Moon will glow in shades ranging from an intense orange or red or, depending on the particulate matter in the atmosphere, to a charcoal gray-black. Because the Moon is moving through the north half of the umbra, the upper half of its disk will likely look brighter than the lower half.
  • Partial stage: The Moon’s leading edge re-emerges into the sunlight, ending totality.
  • Penumbral stage: The Moon escapes the umbra and only penumbral shading is detectable. Around 30 to 40 minutes later the Moon will shine in all its dazzling glory again.

5. What affects the colors seen during the eclipse?

The Moon usually turns some shade of red during the totality of a lunar eclipse, but due to the tilt of the Moon’s orbit with respect to Earth’s orbital plane, not all lunar eclipses occur with exactly the same alignment. Sometimes the Moon goes deeper into Earth’s umbra, and sometimes less deep. The depth of the eclipse, plus conditions in Earth’s atmosphere, affect the color and brightness of the eclipsed Moon.

6. Why is this event being called the “Super Blood Wolf Moon?”

Let’s start at the back end of the phrase. The full Moons of different months have monikers in most cultures (think of the Harvest Moon). Among other names, the full Moon of January is called the “Wolf Moon” because the howling of wolves could be heard on cold winter nights.

Lunar eclipses are sometimes referred to as “blood Moons” because of the reddish tinge that bathes the surface of the Moon during totality.

Although technically the term for the Moon near perigee — when it’s closest to Earth in its orbit — is perigee-syzygy, this has popularly come to be called a “supermoon.” Perigee will occur some 14½ hours after the maximum eclipse, and during this event, the Moon will appear to be some 7% wider than average, a practically undetectable difference

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Astronomers use this five-step “Danjon scale” to judge the darkness of the Moon during a total lunar eclipse.
Leah Tiscione / Sky & Telescope

For more information visit – https://www.skyandtelescope.com


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