Full Moon rising over Yosemite
The Dance of the Sun and the Moon
As I walked home from my train I looked up to see the rising full moon loom large above Mountain View California. Turning around, I watched the fat orange ball of the sun sink behind the fog draped Santa Cruz Mountains. Sun set and full-moon rise go together. As I looked back and forth from the sunset dimmed solar disk to the face of the moon I tried to compare their sizes. To me, they looked about he same size. Holding my hand up at arms length to measure the sun and the moon they both seemed to be about half as wide as my little finger.
Changing my point of view I could imagine rising high above the north pole of the earth looking down to see the sun, earth and moon lying on a straight line &endash; a pattern astronomers call syzygy. The earth was in the middle of this planetary sandwich, giving me a view of the sun to one side and the moon to the other. The sun is larger than the moon and further away from me, amazingly, the sun is both 400 times larger in diameter than the moon and also 400 times further away than the moon. This amazing coincidence is what makes the disk of the setting sun look the same size as the disk of the rising moon.
The three points of sun, earth and moon do not lie exactly on a line. This is why there isn't a lunar eclipse every full moon nor a solar eclipse every new moon. The moon usually passes above or below the earth sun line, close enough to make the moon very full or new, but far enough away to avoid the shadow. Tomorrow morning the moon will skim through the earth's shadow in a partial lunar eclipse.
But the moon orbits the earth. In a few days the moon would be on the wane, rising later and later after sunset. Shrinking from full toward third quarter with only one half of the illuminated moon visible from earth. The moon moving away from syzygy, marching across the night sky approaching a rendezvous with the sun. Approaching syzygy again, but this time the order along the line would be Sun, moon and earth.
When this happened next I would be standing in the town square of Amasya, Turkey. Together with a crew from the Exploratorium. Waiting for the last total solar eclipse of the Millennium. Then it would be really important that the disk of the moon looks about the same size as the disk of the sun when viewed from the earth. This near equality in size allows the disk of the moon to totally block the bright photosphere of the sun. Turning night into day in a spectacle called a total solar eclipse.
Those inveterate gamblers, the weathermen, predict that on that day we have an 80% chance of clear skies. Pretty good odds. But weathermen also know that their climate predictions are what is expected, the actual weather is what we will get. So I'll travel to Turkey and wait to rendezvous with the shadow of the moon. Hoping for clear skies so that the Exploratorium can bring a view of the eclipse to people watching the web from all over the Earth.
I'm excited. I've seen three previous total solar eclipses and one annular eclipse. They were primal experiences. Unforgettable. I long to see another eclipse and share it with thousands of people standing around me in Turkey, and millions of people watching on the web.
Visit the Exploratorium web pages for more information on eclipses:
Or visit my page on how to view a partial eclipse safely.
Then wander outside and look for the moon in the morning sky, watch it approach the sun. I'll be watching with you.
Go to the next story.
Scientific Explorations with Paul Doherty
27 July 99