September 2018 Sky-Watch


We have been highlighting visible planets that have come into prominence for the last several months. Jupiter, then Saturn, and last month Mars, have all come into positions of their best views this year, and in the case of Mars, also its best view for sky-watchers in 15 years.

This month the planet Neptune comes into prominence but only at +7.8 magnitude, binoculars or telescope are necessary to find it. It lies among the dim stars of zodiac constellation Aquarius and will be about midway to the zenith in the southern sky by 1 am on September 5th. You may wonder why I am telling you about such an obscure object that few sky-watchers among us will actually be able to see, even with the help of an accurate star chart (Neptune is close to 4th magnitude Phi Aquarii on Sept. 5th). Looking for obscure sky objects is a challenge. Modern skies are often lit-up by man-made lights, such as our western skies are by the lights of Annapolis and Baltimore, even here in rural Kent County.

Obviously our night vision improves the darker it is or the longer we remain in a darkened environment. Inside our planetarium people often comment that they can pick out more details below the dome after being in the dark for 10 or 15 minutes. But the real trick is to check for color in a darkened surroundings.

I often did a lesson with my students where I slowly darkened the planetarium as they tried to discern color in the dimming light. Photopic vision gives humans sharp colorful images whenever there is enough light. Less distinct grayish images come from our scotopic vision.

Scotopic vision kicks in when we look at a distant galaxy or stat cluster through a telescope. Our maximum acuity then is only 20/200; which is legally blind. So we never pick up the startling detail of of the galaxy that we get in magazine photos; and we cannot detect color.

Our photopic vision can detect color in most of the top 25 brightest stars as seen fro m Earth. In the dimmer 3rd and 4th magnitude stars we see no color. But if we look at these dim stars with a telescope, which gathers more light than our eyes, and makes that light brighter, we can see their colors.

It is worth every expense to get to a remote desert, a tall mountain, a flat open plain, or out to sea; far from artificial lights, and observe the sky. Not only do we see more stars, but color appears, along with finer detail of our colossal Milky Way, even without optical aid.

As for out bright planets this month, start due west for Venus, 1/2 hour after sunset and 10 degrees above the horizon (you will need a clear view to the horizon); though at magnitude –4.8 Venus is as bright as it gets on September 21st. Jupiter lies about 25 degrees left of Venus on September 1st and 14 degrees from it on the 30th. On September 12th, the slim crescent Moon will appear 9 degree above Venus and 16 degrees right of Jupiter. On September 13th, the Moon will be 4 degrees to Jupiter’s upper right. Jupiter shines at –1.9 magnitude all month.

About 45 degrees left (east) of Jupiter is Saturn at magnitude +0.4 among the star of Sagittarius, right at the center of our view into the middle of our Milky Way galaxy. Binoculars and telescopes will pick out many nebulae and star clusters in the direction (south/southwest); and a view of Saturn’s rings through the lens of a telescope is a jewel.

Just left of Saturn is Mars at magnitude –2.1, still bright in its reddish/orange color. Surface details are still easy to pick out through a telescope, including the southern polar ice cap and dark and light areas near the equator.

The autumnal Equinox arrives at 9:54 pm EDT on September 22nd, when the Sun appears to cross the celestial equator moving south (lower) along its annual apparent path against our sky as seen from Earth. The apparent path is called the Ecliptic. Night and day length are nearly equal around the Equinox, but day length steadily decreases as we move into the fall season heading toward the end of the calendar year.

by Dennis Herrmann

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