Cumberland Times-News

Bob Doyle - Astronomy

October 27, 2012

How far away can you actually see?

At night, the most prominent object sky in view is our moon. Light takes 1.25 seconds to travel from the moon’s surface to our eyes.

This has been directly verified by four corner reflectors left on the moon by the Apollo astronauts. A four cornered reflector shots back light in exactly the same direction as where it came from. So a quick pulse of a powerful laser beam shot from a telescope on Earth can be used to determine the moon’s distance to within inches.

 A laser pulse from Earth at the moon will return to Earth in 2.5 seconds, traversing twice the moon’s distance. (First travelling from the Earth to the moon and the return trip (from the Moon to the Earth)).

The speed of radiation is 300,000 kilometers per second. So our moon is 375,000 kilometers away or about 240,000 miles. This is the farthest distance that human beings have travelled from the Earth.

 Presently, there is a very bright planet in the eastern evening sky in the late evening. You can tell that it’s a planet by its steady light.

This is Jupiter, the largest planet in our solar system. Jupiter’s distance from the sun is about five times as far away as the Earth.

So when Jupiter lines up with the Earth on the same side of the sun, Jupiter’s distance from us is about four times the sun’s distance. (Five Earth-sun distances minus the Earth-sun distance).

The sun’s light takes about 500 seconds to travel from the sun’s surface to the Earth. So Jupiter’s light would take four times as long or 2,000 light seconds. Since light travels 300,000 km/sec, the Jupiter-Earth distance would be 600 million km or about 373 million miles.

So as we view Jupiter, we are actually seeing the giant planet as it was 2,000 seconds ago or 33 and 1/3 minutes ago.

Now in the western evening sky is the bright star Deneb, the top star of the Summer Triangle. Deneb’s light takes about 2000 years to reach us.

A light year is about ten trillion km or 6 trillion miles. (A trillion is a million times a million). This is a distance of 20,000 trillion kilometers. So as we look at Deneb, we are seeing light from this star that left 2,000 years ago, during the life of Christ.

The record distance that one can see can see across is that of the Andromeda galaxy, a larger spiral galaxy than our own Milky Way.

The Andromeda galaxy is the largest galaxy in our Local Group, which includes our Galaxy, Andromeda and several dozen smaller galaxies.

Seen as a faint smudge off Andromeda’s knee on dark fall and winter evenings (away from city lights), Andromeda’s light takes 2.2 million years to travel to our eyes.

So when we sight this galaxy, we are viewing its light as it shone 2.2 million years ago, about the time that early hominids began to realize the advantage of staying upright. (To spot predators or to see animals they could spear.)

Now during the day, we view the sun, 500 light seconds away. So we see the sun as it appeared about eight minutes ago (500/60 = 8.33 minutes). This is a distance of 150 million kilometers or 93 million miles.

As far as viewing along the Earth’s surface, there is a nice formula to calculate the distance to the horizon. It is Distance in miles = Square Root (Your eye height in feet times 1.5). A person a little more than 6 feet tall standing on the edge of the ocean could see up to Square Root (9) = 3 miles away in the ocean.

Suppose you go on a cruise where you can view the ocean from a height of 60 feet above the ocean surface. Then you can see out to Square Root of  60 times 1.5 = Square Root of 90 = about 9.5 miles.

Sky sights this coming week

Tomorrow is the night of the Hunter’s Moon, a full moon that offers extra evening moonlight through early November. Nearby the moon you will see the very bright planet Jupiter. While the two objects look close, Jupiter is about 1,500 times farther away than our moon.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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