The three-dimensional, starry sky

When we gaze at the sky overhead, everything seems to be the same distance away from us, except perhaps the sun and the moon. Yet, the starry night is the most three-dimensional of all.

Often we read that a certain planet is in a certain zodiacal constellation. Not so: all planets pass in front of the stars. For example on March 5-6, the moon, at 240,000 miles from us, will be passing above Mars when it is 134 million miles from Earth, and at the same time Mars is above the star Aldebaran, which is 390 trillion miles away.

Because distances of stars number so many miles, astronomers use a different unit of measure to denote their distance: the light year. This really is a more meaningful number because it also tells us how long ago the light was emitted from the surface of the star. For example, Aldebaran is 65 light years away. This means that the light shining into our eyes tonight left the star 65 years ago. That beam of light has been traveling at the incredible speed of six trillion miles per year for 65 years. By contrast, the light from our sun is only eight light minutes old.

When we look beyond the sun, moon and planets of our solar system, we are looking out into our Milky Way galaxy of stars. Our sun is just one of those stars, and it is the closest to us. This time of year we can easily view many of the closer stars in the southwestern sky around 8pm in mid-March. Use the star map to locate the stars, and then imagine that you can see them in three dimensions by using this list of their light-year distances:

Our Milky Way is made up of stars of different sizes and temperatures. Betelgeuse and Rigel are much farther away than the other stars on the list, yet they are very brilliant. Betelgeuse is so bright because it is a huge star. Rigel is bright because it is an extremely hot star.

Our Milky Way galaxy is also adorned with stars clustered together as in a diamond brooch. We can see one cluster of fairly young stars by following the three belt stars of Orion beyond Aldebaran to the little bunch of stars that look like a tiny version of the Little Dipper. This young star cluster is the Pleiades star cluster. Eventually, these stars will migrate apart. The stars of the Pleiades are about 400 light years away. 

Our galaxy also contains clouds of gas and dust in which new stars are forming. Look at the beautiful Orion Nebula in binoculars or a telescope. You can find it by looking beneath the line of three stars between Betelgeuse and Rigel. These stars form the belt of the hunter, Orion. Below the belt stars is Orion's sword. This is where you will see the Orion Nebula. This nebula is 1,500 light years away.

From our very close moon, out to the planets, beyond to the stars, and finally out to the distant Orion Nebula, we can certainly see that our night sky is quite three-dimensional. 

Sirius 9 
Procyon 11 
Pollux 34 
Capella 42 
Aldebaran 65 
Betelgeuse 522
Rigel 733

Sky Calendar, March 2006

(also use the accompanying star chart)

Sunday, March 5: Moon passing between the Pleiades star cluster and Mars

Monday, March 6: Moon passing between Capella and Aldebaran 

Tuesday, March 7: Moon passing between Betelgeuse and Capella

Wednesday, March 8: Moon passing below Castor and Pollux (the Gemini twins)

Thursday, March 9: Moon passing between Pollux and Saturn

Friday, March 10: Moon passing above Saturn

Saturday, March 11: The red planet, Mars, is passing to the right of the red star Aldebaran. It will be interesting to compare the two because Mars reflects sunlight off of its ruddy red surface. Aldebaran shines its own light. Mars is much smaller but also much closer to us. 

Tuesday, March 14: Full moon rising during a penumbral eclipse. Look for the full moon low in the east about 7pm. The moon will be receiving only partial sunlight.

Monday, March 20: On this Spring Equinox day, the sun rises due east, is 70 degrees up in the south at midday and sets due west. Tonight the moon passes just above Antares.

Saturday, March 25: Moon passing below Venus in the dawn sky.

Wednesday, March 29: New moon and total solar eclipse visible in North Africa and the eastern Mediterranean (not visible in Mexico).

Phyllis Burton Pitluga is Astronomer Emerita of the Adler Planetarium and Astronomy Museum, Chicago, and is now a San Miguel resident. You can reach her at ppitluga@adlerplanetarium.org