The Celestial Sphere

By Matthew Johnson

The upcoming Perseid meteor shower may prove to be one of the best meteor showers of the year. At its peak on the night of Aug. 11, it may display as many as 60 to 100 meteors per hour.

Comets leave behind particles as they travel through our solar system. The tiny debris left over from the Swift-Tuttle comet produces the Perseid meteor stream when the Earth in its orbit crosses the comet’s old orbital path. The particles enter the Earth’s atmosphere at the combined speed of 30 to 40 miles (about 60 kilometers) per second, a speed faster than 100,000 miles per hour. A cometary particle the size of a grain of sand encountering the atoms that make up our upper atmosphere will glow from the friction and burn up. This also ionizes the air molecules, and their glow adds to the bright meteor trail.

The Perseids are named after the constellation Perseus because the direction (the radiant) from which the shower seems to appear lies within the constellation Perseus. If one traces the origin of the meteors in the sky, almost all originate from within Perseus.

To view this magnificent event, look above the northeast horizon below the constellation Cassiopeia beginning around 11 p.m. on Aug. 11. We are fortunate this year in that the moon will be only a sliver in the sky that night, so its faint light will not obscure viewing even the faintest meteor trails.

Stars and constellations:

Look upward to the zenith on any clear night in August and locate Vega, one of the brightest stars in the summer celestial sphere. Vega, in the constellation Lyra, forms a key part of the what is termed the summer triangle. This group of three bright stars includes Vega along with two other bright stars—Deneb, in the constellation Cygnus, and Altair, in Aquila. The summer triangle is easily discerned from the myriad of surrounding fainter stars. Vega with its slight blue halo has been called the Sapphire of Summer and the Queen Star because of its lofty position at the sky’s zenith. An inexpensive planisphere, which displays the major stars and the constellations at any hour and night of the year, is always recommended when one looks up to the heavens.

Planets:

This August the planet Mercury is lost in the sun’s glare, and Mars will be mostly lost because of its proximity to the sun. Mars may be visible just below the tiny crescent moon the night of Aug. 9. Look west about 30 minutes after sunset to try to locate Mars. Venus will continue to brighten all month and is best observed on the 10th, when it will be to the left of the thin moon, and on the 11th, when it can be located as the brightest object directly below and slightly to the right of the crescent moon. Again, look to the western horizon 30 to 40 minutes after sunset to view Venus and the moon.

The two largest planets, Saturn and Jupiter, both rise around sunset in the southeast and are visible all night long. The best viewing of Saturn will be on the Aug. 20 when it will be directly above the large moon, and Jupiter will be best located the evening of Aug. 21 when it will be above and slightly to the left of the moon. Look to the southeast about 40 minutes after sunset to view these planets. Turn a pair of binoculars toward them and you will be able to view Saturn’s rings and four of Jupiter’s moons, the same four moons that Galileo observed 411 years ago when he turned his telescope to the celestial sphere in the year 1610.

Phases of the moon: 

Aug. 8: New moon: no visible moon. The best time of the month to observe faint objects such as distant galaxies and star clusters, since there is no moonlight to interfere with viewing faint objects.
Aug. 15: First quarter: right half of the moon is illuminated.
Aug. 22: Full moon.
Aug. 30: Last quarter: left half of the moon is illuminated.

This month’s full moon was known by early Native American tribes as the Sturgeon Moon because the large sturgeon fish of the Great Lakes were more easily caught at this time of year. It has also been called the Green Corn Moon and the Grain Moon.