Visions of Earth through the Yutu Rover’s Eyes

by Bob King on December 19, 2013

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Earth eclipses the sun from Chang'e 3's location in the Sea of Rains on April 15, 2014. At the same time, we'll see a total lunar eclipse from the ground. Stellarium

Earth eclipses the sun from Chang’e 3′s location in the Sea of Rains on April 15, 2014. At the same time, we’ll see a total lunar eclipse from the ground. Stellarium

Last night I used my telescope to eye-hike the volcanic plains of the Sea of Rains (Mare Imbrium) where the Yutu rover and lander sit beneath a blistering sun. With no atmosphere to speak of and days that last two weeks, noontime temperatures can hit 250 degrees Fahrenheit (122 C) . That’s hot enough that mission control at the Beijing Aerospace Command and Control Center has decided to draw the shades and give the rover a nap from science duties until December 23 when things cool down a bit.

While studying the subtle gray hues of the Imbrium lava flows I got to wondering what the sky might look like if I could don a spacesuit and visit the landing site “where the skies are not cloudy all day” (to quote a famous song). With no atmosphere to speak of, stargazing can be done both day and night on the moon though I suspect it’s better at night when there’s less glare from your surroundings. Night, defined as the time from sunset to sunrise (no twilights here), lasts about 14.5 Earth days. Days are equally long.

Lunar landscape photographed by the Chang'e 3 lander on Dec. 15, 2013. Credit: CCTV

Lunar landscape photographed by the Chang’e 3 lander on Dec. 15, 2013. Credit: CCTV

 

From Yutu’s point of view, it’s very nearly lunar noon today (Dec. 19) with the sun halfway up in the southern sky.  Looking at the map of the sky from the lander’s location, you’ll see a few familiar constellations and one very familiar planet – Earth!

Phases of the moon and Earth are complementary. When the moon is full, Earth's a crescent. This map shows the Earth in Capricornus on Dec. 20 as thin blue crescent. Stellarium

Phases of the moon and Earth are complementary. When the moon is full, Earth’s a crescent. This map shows the Earth in Capricornus on Dec. 20 as thin blue crescent. Stellarium

Today Earth appears as a very thin crescent a short distance to the left or east of the sun. Because the moon takes just as long to rotate on its axis as it does to revolve around the Earth, the same face of the moon always faces our planet. Because the two are in synchrony, astronomers call it synchronous rotation.

From the perspective of someone standing on the moon, Earth stands still in one spot of sky throughout the 29.5 day lunar day-night cycle. Well, not perfectly still. Because the moon’s orbit is inclined about 5 degrees to Earth’s orbit and its speed varies along its non-circular orbit, Earth describes a little circle in the lunar sky about 10 degrees in diameter every four weeks.

As the sun slowly moves off to the west, our blue planet remains nearly stationary from Yutu’s perspective and undergoes all the familiar phases we see the moon experience back here on Earth: an evening crescent to start followed by a first quarter Earth, Full Earth last quarter and finally, New Earth. I like the ring of that last one.

The lunar landscape at the rover's location is bathed in pale blue light on Dec. 31, 2013 during "Full Earth". Stellarium

The lunar landscape at the rover’s location is bathed in pale blue light on Dec. 31, 2013 during a Full Earth. Stellarium

Yutu and the lander will see the sun drift to the west while Earth moves east, rises higher in the lunar sky and putting on the pounds phase-wise. Today Earth’s glides across the border of Sagittarius into Capricornus. The next Full Earth happens on New Year’s Eve when the sun is directly opposite the Earth in the lunar sky.

Full Earth always happens around local midnight or about one week before sunrise during the long lunar day. On the moon the sun is up for about  two weeks and then disappears below the horizon for another two weeks before rising again.  At Full Earth time, the sun remains hidden around the lunar backside. When the nights are blackest, the bright ball of Earth spreads a welcome blue glow over the desolate landscape.

Earth covering the sun with a flash of the "diamond ring effect" just before total solar eclipse on April 15 and Oct. 8 next year. Stellarium

Simulated eclipse of the sun by the Earth just before totality on April 15 and Oct. 8 next year. On both dates, we’ll see a  total lunar eclipse from the ground.  Stellarium

Things really get interesting during lunar eclipses when the moon moves behind the Earth into the planet’s shadow. The next one’s on April 15, 2014. Here on the ground we’ll see the moon gradually munched into by Earth’s  shadow until totality, when sunlight from all the sunrises and sunsets around the rim of the planet are refracted by the atmosphere into the shadow, coloring the moon a coppery red.

Two pictures of the ring of sunset-sunrise fire around the Earth as it totally eclipsed the sun from the moon. Credit: NASA

Two pictures of the ring of sunset-sunrise fire around the Earth as it totally eclipsed the sun from the moon. Credit: NASA

Yutu will see just the opposite. Looking back toward the Earth from inside its shadow, the rover will witness a total eclipse of the sun by the Earth. If by some wonder the Chinese are able to photograph the event, we’ll see photos of the black ball of Earth rimmed in red fire from sunset and sunrise light refracted by our atmosphere. My interpretation using sky mapping software only hints at the wonder of the scene. Beijing Aerospace, if you’re reading this, please make it happen.


Earth eclipses the sun filmed by Japan’s Kaguya lunar orbiter. There are really two eclipses here – the Earth eclipsed by the limb of the moon at the video’s start followed by the solar eclipse.

On two other occasions, our robotic emissaries have photographed solar eclipses from Luna. NASA’s Surveyor 3 snapped a couple crude pictures of the April 24, 1967 eclipse from inside a crater in Mare Cognitium, the Sea that has Become Known. Japan’s orbiting Kaguya probe did the job much more eloquently on video during the February 9, 2009 penumbral lunar eclipse. In a penumbral eclipse (seen from Earth) the moon misses Earth’s dark inner shadow called the umbra, passing only through the outer penumbra, but because the Earth is three times larger than the sun (seen from the moon), it easily covered the sun completely in the complementary total solar eclipse.

And the best thing about watching eclipses from the moon? Guaranteed clear skies!

About 

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. Every day the universe offers up something both beautiful and thought-provoking. I also write a daily astronomy blog called Astro Bob.

Sorin Lungu December 19, 2013 at 11:32 AM

I found this very interesting, and cannot wait for more pics from Yutu…maybe we can make a difference with the NASA’s pics
And even more amazing is that not many people are interested…says a lot about the world we live in today…sad but true

ethanol December 19, 2013 at 11:35 AM

Cool, hope it lasts that long. However, I have one quibble: you said:

“If by some wonder the Chinese are able to photograph the event, and it won’t be easy since it happens during the coldest part of the lunar night”

It seems that this eclipse will most definitely happen during the day, near noon in fact, although temperatures will certainly drop temporarily.

IVAN3MAN_AT_LARGE December 19, 2013 at 1:57 PM

Full Earth always happens around local midnight or some two weeks before sunrise during the 28-day-long lunar day.

Er… actually, Bob, I think that should be about one week, not “two weeks”: the Sun would have set about one week earlier by the time the Earth appears full at lunar midnight, so about another week would pass before the Sun rises again.

Bob King December 19, 2013 at 3:05 PM

Ivanman and ethanol,
Thanks for pointing out the errors. I was shifting back and forth between moon and Earth, I slipped! Mistake now corrected. Thanks again for alerting me.

Olaf2 December 19, 2013 at 2:00 PM

I think it is also interesting to see what happens when the sun goes down or up.

Please show more pictures! :-)

Bob King December 19, 2013 at 3:10 PM

Thanks for pointing out the errors. I was shifting back and forth between moon and Earth so often, I slipped up! Mistake now corrected. Thanks again for alerting me.

Olaf2 December 20, 2013 at 1:20 PM

Silly me. I was wondering why they landed during almost full moon and not when there was first quarter moon so they had more days to do research. But the position they landed is not in the centre seen from Earth so from the point of the lander there is still lots of sunlight when the moon goes to last quarter moon.

Hang Wong December 20, 2013 at 1:53 PM

According to Xinhuanet, the hibernation of Yutu was ended ahead of schedule. Moreover, panoramic photo captured by the lander is released. (Only shown in CCTV news video… http://news.cntv.cn/2013/12/20/VIDE1387538287920838.shtml )

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