This is Dudley Observatory’s Skywatch Line for Friday, September 22 through Sunday, September 24, written by Sam Salem.
On Friday, Sun rises at 6:43am and sets at 6:53pm; the Waxing Crescent Moon sets at 8:29pm. The new Moon occurred on Thursday at 1:30am.
Jupiter, at –1.7 magnitude, is the twilight-only planet this weekend. It sets roughly an hour after the Sun. As twilight begins to fade, Saturn, at 0.5 magnitude, is about 20 degrees high in the southwest. Venus, at–3.9 magnitude, rises before 5 am. Next to appear is Mars, at 1.8-magnitude. The red planet is a relatively faint dot that clears the east horizon at around 5:30 am. A few minutes later, Mercury, appears at magnitude –1.1.
The fall equinox occurs on Friday at 4:02 pm. On equinox, the Sun rises due east and sets due west. An equinox happens when the ecliptic, or Sun’s path, intersects the celestial equator, the imaginary line above Earth’s equator. An equinox occurs when the Sun crosses the celestial equator. No matter where you are on Earth, the celestial equator intersects your horizon at due east and due west. At its highest point in your sky, the celestial equator appears high or low, depending on your latitude. On Equinox day, Sun is midway between the Sun’s lowest path across the sky in winter and highest path across the sky in summer.
Use the Big Dipper to find Polaris, the North Star. Draw a line between the two outer stars in the bowl of the Big Dipper. A line from the two outermost stars in the bowl of the Big Dipper points to Polaris. Polaris marks the tip of the handle of the Little Dipper. The northern sky is a large clock, with Polaris at its center. The hour hand is a line drawn through Dubhe and Merak, the two pointer stars of the Big Dipper. The stars make a full circle in 23 hours 56 minutes instead of exactly 24 hours. Therefore, if you look at the same time each evening, the Big Dipper will appear just a little bit lower in the northwestern evening sky. A month from now at mid-evening, the Big Dipper will be noticeably lower in the northwest.
The faint cone of light known as the zodiacal light extends from the east horizon, upwards along the ecliptic. This weekend is free from moonlight, which makes it one of the best times of year for viewing the zodiacal light. The pale glow of the zodiacal light arises from sunlight scattered by dust particles residing within the inner solar system. Look for an amorphous, cone-shaped luminance in the east before morning twilight begins. Usually, the zodiacal light is only about as bright as the winter Milky Way, which is why you’ll need a dark, clear sky to detect it. You might find the zodiacal light easier to capture with a camera than your eyes. Use a wide-angle lens, aim towards the east, and experiment until you find a combination of aperture, ISO, and exposure time that yields the best results. Set the camera’s white-balance to “daylight” to capture the most natural colors.
Saturday marks the anniversary of Neptune’s discovery. On September 23 1846, the German astronomer Johann G. Galle discovered Neptune after only an hour of searching, within one degree of the position that had been computed by the French astronomer Urbain-Jean-Joseph Le Verrier.