Whether it signals the end of vacation, the start of a new school year, or the swapping of sandals for shoes, September is often met with reluctance. But this year, even the planets won’t know whether they’re coming or going!
Every day, the planets, just like the stars, the Moon and the Sun, can be seen moving from east to west across our sky. In reality, we are the ones moving as a result of Earth’s rotation on its own axis. If we could stop our planet from spinning, the only noticeable movement against the background stars would be the planets travelling slowly around the Sun.
Planets generally follow an eastward path through the stars, or to the left for observers in the Northern Hemisphere, but periodically appear to reverse direction and travel backward toward the west. Up until the Renaissance, this retrograde motion of the outer planets was a controversial issue in our models of the Universe; those who defended geocentrism and who believed that the planets moved in perfect circles had a hard time explaining those bewildering objects in the sky. Let’s try an exercise to help make sense of what happens...
Hold your right arm straight out to the side, parallel to the ground, with your thumb pointing up; this will represent a superior planet, let’s say Jupiter. Without moving your body, turn your head so that you’re looking at your thumb, then close one eye; your open eye will represent the Earth. We’re all set now.
Keeping your head still, simulate Jupiter’s revolution by moving your right arm inward until your thumb is directly in front of your body. Notice that it moves from right to left. Make note of whatever object appears behind your thumb: a painting on the wall, a window, or a tree—the farthest object possible.
Now, without moving your thumb, simulate Earth’s rotation by turning your head 180°, or until you’re facing left, and pay attention to the apparent movement of your thumb relative to the background object: It appears to reverse course and move toward the right! This illusion of backward motion is called retrograde.
Now, if you begin moving your right arm again, you’ll once again see your thumb moving to the left.
In reality, planets travel in a continuous, simultaneous manner. If you have good coordination and a keen sense of observation, you can try moving your thumb and head at the same time—just make sure to move your thumb very slowly. You can also switch things up by bringing your thumb closer to your face, to simulate a planet nearer to us, such as Mars. The retrograde motion will be even more obvious. But in all cases, retrograde motion would not exist if the Earth did not travel around the Sun (which you can imagine as being your spine).
That’s 1-0 for heliocentrism!
Planetary back and forth in the September sky
Brilliant Jupiter dominates the sky at dusk. The planet is very easy to spot thanks to its dazzling shine and culminates slightly more than 20 degrees above the southern horizon by 9 p.m. at the beginning of September (7 p.m. toward month’s end). The slightly fainter Saturn closely trails Jupiter, appearing about 8 degrees to its left, or a little less than the width of a fist at arm’s length. As their retrograde motion comes to an end, the two planets will appear to halt in relation to the stars of Sagittarius before resuming their slow eastward course, on September 13 for Jupiter and on September 29 for Saturn. On the evenings of September 24 and 25, your gaze will likely be drawn to the first quarter Moon meandering through this region of sky.
Mars and Earth will be closest to each other on October 6. Throughout September, the Red Planet will increase in magnitude (from –1.8 to –2.3) and in apparent size (from 18.9 to 21.6 arc seconds), as well as rise at a more convenient hour (10 p.m. at the beginning of the month and 8 p.m. at the end). Look for a very bright orange point of light on the eastern horizon, among the stars of the constellation Pisces. Initially located near the left arm of the large V depicting the two fish of Pisces, Mars begins its retrograde loop on September 9 and ends up at the V’s right arm by month’s end. But you won’t need the fish to find the Red Planet, which is by far the most brilliant celestial object in this region of sky this year, except on the night of September 5-6 when the waxing gibbous Moon will lie less than half a degree below Mars. Try not to miss this conjunction! Observers in much of the Southern Hemisphere will even have a chance to witness an occultation of Mars by our natural satellite.
Throughout the Moon’s travels
Speaking of our sidekick, the waning gibbous Moon will hang between the Pleiades and Hyades star clusters, affording a lovely sight as they rise at around 11 p.m. on the evening of September 8.
A few days later, on the morning of September 14, the last crescent Moon will meet up with Venus and the Beehive cluster (M44), both of which rise above the east-northeastern horizon after 3 a.m. This close-knit trio will hang within 4 degrees of each other and will easily fit within the same field of view of most binoculars. Venus, in its appearance as the Morning Star, will be well positioned throughout the month for early-bird observers as it sits high above the eastern horizon at dawn, shining at a magnificently bright –4.2 magnitude.
Finally, the new Moon on September 17 will provide an opportunity to reconnect with the deep sky, particularly with the Andromeda Galaxy, which is back in prime viewing position, high overhead and visible all night long at this time of year.