45th Anniversary of the Famous Earthrise Photo

It was on Dec. 24, 1968 that astronauts aboard the Apollo 8 mission orbiting the Moon took one of the most famous images in the history of photography. It's become known as "Earthrise on the Moon," and we will soon be celebrating its 45th anniversary.

Moon chronicler Andrew Chaikin has worked with NASA to make a short video of the complete story of how this image came to be taken, and wrote me about recently. I thought I would share it with all of you -- whether you are an old geezer like me (and were alive in 1968) or whether you are newer to life on Earth and saw the picture later. You can find the video at:http://www.youtube.com/watch?v=dE-vOscpiNc

I love the image and its portrayal of a small, fragile planet Earth isolated in space above the airless surface of the Moon. But since this is the AstroProf's page, I have to add, like a good teacher should, that the name creates a bit of a misimpression.

When we talk about sunrise on Earth, we mean that as the Earth turns, we can see the Sun rise above the eastern horizon. As the day goes on, we see the Sun move through our skies, until, in the evening, it sets below the western horizon. What we see as the rising and setting of the Sun is just the effect of the 24-hour rotation of our planet. The spinning of the Earth causes the whole sky to go around us in a day.

The Moon's spin is much slower than Earth's! It takes the Moon 27.3 Earth days to spin once. And in exactly the same 27.3 days, the Moon also orbits once around the Earth. This "tidal locking" of the two motions, the Moon's spin and its revolution around us, means that the Moon always keeps the same face to the Earth.

If you don't believe or see this, try the experiment with your little brother. Put him in the middle of an empty space, face him, and then go around him in the exact same time as you turn your shoulders in a circle. If you take the same time to go around him as to go around yourself, your little brother will always see the front side of your body and never the back side.

For the same reason, if you stand on the Moon, you will either see the Earth in your sky (if you are on the near side of the Moon) or you will not (if you are on the far side). But the Earth will not rise or set in the Moon's sky the way the Sun or Moon rise and set in our sky.

In the famous picture, the Earth only seemed to be "rising" to the astronauts because the Apollo spacecraft was orbiting the Moon.

But if future visitors to the Moon's surface really want to see an Earthrise, it is possible. The Moon wobbles back and forth just a bit (we call it "lunar libration") and so, if you stand in just the right place on the Moon, you could see the Earth dip very slowly below the horizon and then come up again. (Google lunar libration if you want the technical details.)

For everyone else, just enjoy the image and the video. In these times of strife and mistrust everywhere, it's good to remember that -- in the larger picture -- we are all passengers aboard spaceship Earth, sharing its limited area and limited resources.

Asteroid Passing Close to Earth on Friday

Artist's Impression from NASA/JPL

This Friday (2/15) a small asteroid will fly by the Earth at quite close a distance -- only about 17,000 miles above our planet's surface.   That's closer than some of the satellites that orbit the Earth. Closest passage will be around 11:30 am Pacific time on Friday, but it would not be visible without a telescope even if it were passing during night time.

There is nothing to fear from this flyby -- the asteroid is going to miss the Earth and shoot by us at eight times the velocity of a high-speed rifle bullet.  But it does remind us that we live in what some astronomers call "a cosmic shooting gallery," where chunks of rock and ice left over from the early days of our solar system still fly around and occasionally hit a planet or a moon. (You can see billions of years of such hits if you point a pair of binoculars at the Moon. Having no atmosphere, oceans, vegetation, or teenage children to erode its surface, our Moon preserves every impact as a round crater on its surface.)

This particular asteroid, discovered by a Spanish observatory last February, is given the catalog name of 2012 DA14.  We are not sure of its exact size, although we will learn much more about it as it swings by.  We estimate that it's about 90 to 150 feet in diameter, although it is probably not round, but shaped like a fresh potato (as shown in the JPL artist's conception that accompanies this article.)  If something this size were to hit the Earth at typical near-Earth asteroid speeds, it could take out an entire city, much like a nuclear bomb.  It could wreak devastation even if it never reached the surface, but exploded from the heat of friction in the atmosphere. 

You can understand why astronomers are working hard to catalog every significant-sized chunk of rock that crosses our planet's orbit.  Of course, the smaller the object, the harder it is to see when it's far away. But this way, if one of these larger cosmic "bullets" does have our name on it, we can know about far in advance and prepare.  With enough notice, we might even have the technology to nudge it aside and help it to miss our planet.  This would be the kind of project that would make astronomers and NASA popular for a while!

Happy Winter Solstice (Not Doomsday)

Let me wish all my readers a happy Dec. 21, the day of the winter solstice in the northern hemisphere. Despite the hue and cry of the doomsayers, the Sun rose today and our planet continues to orbit in the usual cosmic order of things. There is no greater or lesser chance of disaster today than any other day.

Today is special, however, for being the day that the Sun makes the lowest arc in our skies and thus gives us the shortest day of the year (as beautifully illustrated on the accompanying photo by Italian astro-photographer Danilo Privato.)

For thousands of years, civilizations throughout the northern half of our planet have planned special celebrations to cheer each other up around the time the day was shortest and the night was longest. They gathered with their tribe, lit a fire, danced and sang to implore their sun-god to return, made sacrifices (like burning a valued food animal), and perhaps gave a present of something they treasured to their loved ones.

Many of our modern traditions began with such winter solstice celebrations. I encourage you to reach out to relatives and friends today in the same way, perhaps sharing the beautiful solstice photo (a composite image made of 43 different exposures during a past solstice day from Santa Severa, Italy.)

Today, as on all other days of our annual journey around the Sun, we have far more to fear from our own actions than from the cosmos. It is a sad fact that the most likely form of local or global doomsday for planet Earth will come from humanity's own prejudices, superstitions, hatreds, and persistent refusal to face facts. In our lifetimes, our planet is probably going to suffer more from our pollution and carelessness than from the actions of the universe. Now that we seem to be surviving any "Doomsday 2012" coming from the skies, maybe we can resolve to do a little better as residents of planet Earth.


(Photograph used by permission. To check out the many astronomical photographs by Danilo Privato, please see his website: www.daniloprivato.com )

To learn more about why the Dec. 21, 2012 doomsday fears had no basis in science, you can check out the Calendar of the Sky website from the University of California, Berkeley Space Sciences Lab -- a site to which I had the pleasure of contributing.  You might start with the essay: 
http://www.calendarinthesky.org/Articles/2012Science/2012ScicenceArticleView/tabid/144/ArticleId/43/ 
and then browse through the menu at left.)