Observing the Moon and Planets

Young Astronomers Blog, Volume 29, Number 14.

Saturday, October 16, 2021, is International Observe the Moon Night, so this is a great time to go outside, look up, and do some observing.

Many amateur astronomers begin their tour of the night sky by viewing the constellations. For more on this, see the Night Sky. Once you become familiar with the constellations and purchase a telescope, you can move on to the Moon.

The Moon

The Moon is a great target for “star gazing.” Or I guess we should call it “Moon gazing.” The Moon goes through phases every 29 ½ days as the Sun shines on different parts of the Moon. The Moon will also rise and set at different times during this cycle (see the table below), so you will need to choose an observing time that corresponds to when the Moon is visible.

The best time to view the Moon is when it is partially illuminated. A full Moon is typically too bright and much of the lunar detail is hidden. If you’re star gazing, as well as Moon gazing, a crescent to quarter Moon is best to keep the stars from being washed out by the Moon’s light. If you’re observing only the Moon, other phases, except a full Moon, are good. In fact, you can follow the Moon’s phases over several nights and focus on different landmarks each night. Find the current phase of the Moon for the Winston-Salem area in the Old Farmer’s Almanac.

You will find the features near the day/night line (the terminator) easy to spot as shadows bring out the mountain peaks and crater rims. A few of the more prominent landmarks are the Maria (aka seas) such as the seas of Tranquility, Serenity, and Rains/Showers, along with the Ocean of Storms. Three of the more noticeable craters are Copernicus (below the Sea of Rains) with Kepler to its west, and Tycho (with its rays) toward the south. If you’d like an English translation of the Latin names, see The Planetary Society’s map of Moon features.

Make sure you have your “moon sunglasses,” that is a moon filter. Even the reflected light shining off the Moon can be bright. A 13% (for a large telescope) or 25% (for a small telescope) moon filter will darken the view and make it much more tolerable, just as sunglasses make it much easier to see on a bright sunny day.

The Planets

After observing the Moon for a while, it is time to view the planets. Five planets are visible to the naked eye most of the time if you know where to look.

Jupiter and Saturn are my favorites telescope targets. They move slowly west to east against the background of stars. I can usually spot the four Galilean moons, Io, Europa, Ganymede, and Callisto, looking like stars off to the side of Jupiter. Sky & Telescope has a website that shows the position of these four moons. Saturn’s rings are visible even through a small telescope. On a good night, I can just make out the Cassini division between the A and B rings. With a bit more concentration I can see two of the cloud bands on Jupiter and Saturn’s largest moon Titan.

Venus is always the brightest planet in the night sky. It goes through a 584 day “synodic” cycle relative to the Earth. It appears as a small white disk when on the other side of the Sun from the Earth. As it crosses behind the Sun (Superior Conjunction) it moves from the eastern horizon before sunrise to the western horizon before sunset. As it moves closer to the Earth, its phases appear, and it becomes a larger and larger object through a telescope eyepiece. It eventually appears as a thin crescent. As it passes in front of the Sun (Inferior Conjunction), it moves from the western horizon before sunset back to the eastern horizon before sunrise. It then slowly recedes from the Earth again going through its phases and finally becoming that small white disk.

Mercury follows a similar pattern with phases like Venus but stays lower in the sky and its “synodic” cycle is only 116 days.

Mars follows a 26-month cycle relative to the Earth. Every twenty-six months, Mars is in opposition to the Sun. This is also its closest approach to the Earth, and it appears bright in the sky. Mars has a somewhat elongated orbit, so some oppositions are closer than others. Perihelic opposition (2003, 2018, and 2035) can be as near as 35 million miles. Aphelic opposition is as far as 60 million miles.

Uranus and Neptune are harder to find. They aren’t visible to the naked eye and your best bet is to use one of the many astronomy apps with augmented reality. The good news is neither planet moves very quickly against the background of stars. Uranus is currently (late 2021) located to the south of Aries and (north) west of the Pleiades. Neptune (late 2021) is just to the south of the Circlet of Pisces.