Night Sky—Our changing night sky
Summer is a great time to go out and observe the sky, though we might have to stay awake a bit longer since sunset is later in the evenings. You may have noticed that the star patterns and constellations have changed since the winter months. For example, Orion, the great hunter, adorns the cold winter sky. Spring brings Leo, the lion, into view. Scorpius, the scorpion, dominates the southern summer sky. And the Great Square of Pegasus calls for our attention on fall evenings.
Why do the star patterns change with the seasons?
Watch the night sky on any night from dusk to dawn and you'll notice certain stars rising from the eastern horizon. They move across the sky during the night, finally setting on the western horizon by dawn. But, go out at the same time each evening and note the location of the stars and you will see a more subtle change in their position. Stars that were low over the western horizon during the early evening hours will, within a matter of a few weeks, disappear entirely from our view. Taking their place will be groups of stars that were higher up in the sky a few weeks earlier. In fact, it would seem that with the passage of time, all the stars gradually shift westward while new stars move up from the eastern horizon to take their place.
Earth rotates on its axis causing night and day. At the same time, we are also orbiting the Sun. Each night we have a slightly different perspective of the night sky, since the Earth moves a short distance in its orbit every day. If we were to synchronize our clocks using the motions of the stars as a reference we would discover that the Earth would complete a single turn on its axis not in twenty-four hours, but in twenty-three hours and fifty-six minutes. Because of our changing position in our orbit around the Sun and the rotation around our axis, the stars appear to rise, cross the sky, and set four minutes earlier each night. Those four minutes each night add up to two hours every month. Over the course of a full year, those four minutes make twenty-four hours difference in rise and set time.
When you put all of this together, it means that six months from now, the stars you see in the sky this evening will rise twelve hours later. They would then be visible during the day if we could see them in the bright glare of the Sun.
Summer is not just for star watching. If you watch the twilight sky from forty-five to ninety minutes after sunset, or forty-five to ninety minutes before sunrise, you may see a few "moving stars." They are most likely artificial satellites. The biggest, and one of the brightest of all, is the International Space Station, and seasonal circumstances make now a great time for anyone to spot the ISS and its orbiting cousins. If you go out and carefully study the sky near dusk or dawn, you should not have to wait more than fifteen minutes to see an Earth-orbiting satellite.
Satellites shine because they reflect sunlight. A satellite entering the Earth's shadow immediately vanishes from view and pursues an unseen path until it again emerges into full sunlight.
Around five thousand satellites have been launched into Earth orbit. About two thousand are still there. There are thousands of smaller pieces of junk, most of which are too small to see, also circling the Earth. Depending on who’s counting, anywhere from one hundred to three hundred satellites can be seen with the unaided eye. These are generally more than about twenty feet in length and anywhere from one hundred to four hundred miles above Earth.
The ISS is by far the biggest man-made object circling the planet. Its solar arrays span 240 feet. The main modules are 146 feet long, and it stands as tall as a nine-story building. Traveling around the Earth at eighteen thousand mph at an average altitude of 240 miles, the station can appear to move as fast as a high-flying jet airliner. It sometimes only takes about four to five minutes to cross the sky. To the unaided eye, it could easily be confused with an airplane.
From now through the beginning of July, nights are shortest and the time that a satellite in a low Earth orbit can remain in the sunlight is greatest for the northern hemisphere. How do you know when and where to look for these objects? I use the Web site www.heavens-above.com. Once you register with the site (it is free), you need to pick your location. I used the following latitude and longitude for my home in Placitas: 35 degrees, 19 minutes, 46 seconds north by 106 degrees, 27 minutes, 7 seconds west. Those coordinates should be close enough for most Signpost readers to use.
The Website will generate a list of all of the objects that you can see that evening and will tell what they are, when to look, and in what direction to look. As a bonus, when you know the name of the satellite you are watching, you will sound like an expert to those who don’t read this column.
On the morning of June 8, Venus will make a transit of the Sun as seen from Earth. Unfortunately, the Sun will not rise in New Mexico until after the event is over. If you live farther east, you can watch (using a safe method to view the Sun) Venus move across the face of the Sun.
This is a rare occurrence. Because of the inclination of our orbits and the way the planets line up, it has only been observed previously in 1639, 1761, 1769, 1874, and 1882. In fact, between the years 2000 B.C. and A.D. 4000 there are only eighty-one Venusian "transits," as astronomers call them. Every 243 years, there are two opportunities to witness this event. Our next chance will be on June 6, 2012. If you miss that one, you will have to wait for December 11, 2117.
The entire June 8 transit will last just more than six hours and will be visible in some form across approximately three-quarters of our planet. The beginning will be visible from northern and western portions of Alaska and all of Asia. The end of the transit will be seen from the east coast of the United States to about eastern Texas. Unfortunately for those of us living in the western United States and Hawaii, the Sun will be below the horizon when the show takes place.
Venus is setting earlier in the evening and will be below the horizon most of the month.
- Mars is also low and dim on the western horizon and will setting shortly after 10:00 p.m. Look for the crescent Moon to the right of Mars on June 19 one hour after sunset. You will also find Pollux and Castor (the twins) just to the upper left of the Moon.
- Jupiter is still big and bright west-southwest. One hour after sunset on June 23, find the Moon just above bright Jupiter.
- Saturn is exiting the night sky and will not be visible.
- The full Moon is on June 3. If the full Moon looks a bit larger tonight, it is because it is as close to the Earth as it ever gets (221,983 miles) making it appear to be 8 percent larger than it will on June 17 when the Moon is at its maximum distance (252,633 miles). June 17 is also the New Moon.
Do you have a question I might be able to answer in an upcoming column? Would you like to comment on an article? Send e-mail to me at email@example.com.