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Night Sky

September 2010

—Charlie Christmann


I was talking to some people about the night sky recently, and the question of distances in the universe came up. It is hard to convey just how vast even our own galaxy, the Milky Way, really is. But, then, come to think of it, our solar system is expansive in itself.

Solar System

Let’s take the brightest planet in the evening sky as our first example. Venus orbits the sun between Mercury and Earth at an average distance of 67.1 million miles. The closest it gets on average to Earth is 25.8 million miles and the farthest (when it is opposite the sun) is 160 million miles. Mars, at its closest on average, is 48.6 million miles and when it is opposite the sun, it is 233.4 million miles. If we try to get to Neptune, it’s a long 2,701.4 million miles at it’s closest.

Even if we discount planetary motions, in a standard jet, flying at 600 mph nonstop, it would take at least 4.9 years to get to Venus. Mars would be a 9.25-year trip. Then again, a journey to Neptune would be 514 years. And you thought transatlantic flights are long…
Trips to Mars actually only take 8 months, when we take orbits into account, even though the distance traveled is much greater than a straight-line trip. But that is because the velocity of Earth added to the kick from the rocket boosts a spacecraft to 66,500 mph—more than 110 times faster than a jet.

Even the distance to the sun from Earth is almost incomprehensible. At 92.9 million miles, the sun is still hot enough to keep Earth warm and comfortable. Yet, if the sun suddenly went dark, it would take about 8.5 minutes before we knew anything happened. Your fast jet will take 17.7 years to fly between the Earth and the sun.

Galaxy

Our nearest star, Alpha Centauri, is not a star, but really a star grouping of three stars. The closest of these is Proxima Centauri. Sorry, the Alpha Centauri system is not visible from the Northern Hemisphere. It is only 25 trillion miles, or 4.22 light-years, away. When distances get so large, we use a new unit of measure—the distance light will travel in one year. A light-year is about 5,878,500,000,000 miles (that’s trillions of miles). This means that a laser fired at Proxima Centauri would take 4.22 years to reach the star. In that jet plane, try 4.66 million years to get there nonstop.

Our 5 nearest stellar neighbors are:

Name                          Distance in Light-Years              Jet Flight Time (millions of years)
Proxima Centauri                 4.24                                        46,600
Alpha Centauri A                  4.35                                        42,500
Alpha Centauri B                  4.35                                        42,500
Barnard’s Star                                   5.98                                        58,583
Wolf 359                                7.78                                        76,167

Current estimates put us at about 25,000 light-years from the center of our galaxy (good thing, too, a super giant black hole is lurking there). Do you really want to know the jet travel time? OK, it’s only a whopping 24,500,000 million years of travel. If we could travel as fast as a Mars probe, the time would be cut down to 2,210,626 million years. And that is less than halfway across our galaxy.

Intergalactic

Now, we get to some really big numbers. The distances among the galaxies are just too big to comprehend, even when we start with light-year units.

The Canis Major Dwarf galaxy orbits our Milky Way at 25,000 light-years. That’s close enough that it will likely plow through the galactic disk some day. The Sagittarius Dwarf galaxy is next in distance, orbiting at 81,000 light-years (793 million jet years). The Large Magellanic Cloud also orbits our galaxy at 163,000 light-years.

The nearest, non-orbiting galaxy is actually a globular cluster, MGC1, 2 million light-years away (2,237,443 million jet years). Our nearest true spiral galaxy, Andromeda Galaxy (M31), is around 2.56 million light-years distant.

The Andromeda Galaxy and the Milky Way are on a collision course. They are approaching each other at 268,432 miles per hour (0.04% the speed of light). But even at that phenomenal speed, it will take at least four billion years for the merger of the two galaxies to even start!

Now, for the ultimate mind bending distance: Our universe is finite in size. The best measurements available put it at 156 billion light-years across—that’s 9,170,000,000,000,000,000,000,000 (917 septillion) miles. And yes, I’ll give you the jet years: 1,744,672,754,946,727,549.5 years (1.7 quintillion).

The Planets and Moon

  • To find Mercury, look low in the east about 60 to 30 minutes before sunrise after mid-month. Mercury will be the farthest from the sun on the 19th.
  • Though it is starting to get slightly dimmer throughout the month, Venus is still dominant in the west after sunset. Check out the west-southwest 10 minutes after sunset on the 11th. You’ll find a fantastic grouping of the waxing crescent Moon, Venus, Mars, and Spica. To the west is Saturn. Antares is to the south, and Arcturus is above the grouping.
  • Mars can be found low in the west-southwest after sunset through mid-month.
  • Jupiter is rising in the east about 8:30 p.m. early in the month and at 6:30 p.m. late in the month. Its brightness will rival Venus.
  • Saturn joins Mars low in the west-southwest after sunset through mid-month. The two will be tightly grouped all month.
  • Uranus can be found near Jupiter. You’ll need good binoculars or a small telescope. On the 1st, look east-southeast for Jupiter. Uranus will be two degrees to the upper right. Jupiter will appear to slide to the right, away from Uranus each night.
  • The Moon is new on the 8th and full on the 23rd.

 

     

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