Distances in the Universe

This month we will take a look at distances in our universe and how we measure them. The first thing to realize is that space is HUGE. The distance between any two objects in space is more than we can visualize. Our nearest neighbor, the Moon, is about 260,000 miles away. This is the equivalent of 2,600 round trips from Las Cruces to El Paso.

On Nov. 8, Mercury passed in front of the Sun. This image, taken about an hour after the start of the transit, shows Mercury, the black spot on the right, and a large sunspot group that just came around from the back side of the Sun. The entire transit took more than five hours. This image was taken with a six-inch telescope and a full-aperture solar filter.

Not bad, you might say. But the Moon is incredibly close to the Earth in astronomical terms. It is the equivalent of walking out to the garage, yet it took almost 10 years of research and design to develop the Saturn V rocket and the Apollo spacecraft. To travel farther, we will have to develop all-new spacecraft, power systems and environmental systems and push the human spirit to the breaking point just to visit the next closest planet, Mars.

When nearest Earth, Mars is some 40 million miles way, 160 times farther than the Moon. Instead of Apollo's three-day trip to the Moon, Mars is well over a year away, traveling only one way. The other planets are even farther away. The outer edge of our solar system is some 6 trillion miles away, but this is just down the street, in astronomical terms.

These numbers are beginning to get rather large, so astronomers have adopted a unit of measure for these large distances called the light-year. This is defined as the distance that light will travel in a year. So you can compute the distance by taking the speed of light, roughly 186,000 miles per second, multiplying it by 60 seconds per minute, 60 minutes per hour, 24 hours per day and finally 365 days a year to get the distance of the light year, about 5,850,000,000,000 miles.

Light, infrared, ultraviolet, radio waves, x-rays and gamma rays are all part of the electromagnetic spectrum. They all travel at the same speed in a vacuum, the speed of light. You probably have seen the effect of this speed when an interviewer on television is interviewing a guest "via satellite." The geosynchronous satellites are about 23,000 miles above the Earth, the distance at which a satellite will take 24 hours to orbit the Earth, so the satellite stays in the same place over the Earth. When the interviewer asks a question, the signal has to travel about 46,000 miles (23,000 up and then the same distance down). The guest then starts to respond and the signal must traverse another 46,000 miles before you begin to hear the response. The total travel distance is 92,000 miles and at 186,000 miles per second, the pause you hear is half a second. Now consider how far light will travel in a year!

So, using our new measuring stick, Mars is 4 light-minutes away (when close) and the Sun is 8 light-minutes away. Pluto is 4 light-hours away and the edge of our solar system is a whole light-year distant. The nearest star, Proxima Centauri, is "only" 4.2 light-years away. The center of our galaxy is 30,000 light-years away, and the diameter of the Milky Way is 120,000 light-years across. (Remember, the Moon is just 1.3 light-seconds away, the equivalent of 2,600 trips from Las Cruces to El Paso.)

So far we have been talking about objects in our galaxy. The nearest big galaxy is Andromeda (M31), which is a "mere" 2.2 million light years away. A nearby cluster of galaxies, the Virgo Cluster, is 60 million light-years away. Finally, the most distant galaxies that we can see are some 9 billion light years away, or 54,000,000,000,000,000,000 miles.

The Planets for December 2006

Now west of the Sun after last month's transit, Mercury is in the morning sky for the first half of the month before dropping back into the morning twilight around midmonth. This is the best morning apparition of Mercury of the year. Mars, Jupiter and Mercury will all be within a degree of each other on the morning of Dec. 10. Mercury will already be heading back toward the Sun and they will all be very low in the east-southeast about a half-hour before sunrise. This will be the closest grouping of three bright naked-eye planets until 2050. On that day Mercury will be magnitude -0.6 and will be only 5.2 seconds-of-arc across.

Venus is moving into the evening sky, but is still too close to the Sun for convenient observation. You can find it in very low in the southwest during evening twilight. Jupiter, in Virgo, and Mars, moving from Virgo to Scorpius and finally Ophiuchus, are in the morning sky, slowly climbing away from the Sun. They are still too low for detailed observation.

Saturn, in Leo, rises around 10 p.m. and is up the rest of the night. Saturn is moving into its best viewing season, so this would be a good time to take a look at its 19.1 second-of-arc across disc and 43.3 second-of-arc across rings. The rings are now tilted up only 12.3 degrees, with the southern face showing.

Winter begins on Dec. 21, when the Sun reaches the most southerly point in our sky. This will give us the shortest day and the longest night of the year. It marks the beginning of astronomical winter and starts the Sun back northward to warm us into spring. Meanwhile, have a happy holiday season and try to find some time to "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

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