Harvest Moon obscures Uranus’ opposition in late September
The chart shows the southeastern sky as seen from mid-northern North American latitudes. This is the location of the planet Uranus, the seventh planet outward from the sun, which is opposite the sun on the night of September 28. Normally, opposition presents the best time to view a distant planet such as Uranus – except that the almost-full Harvest Moon lights up the sky tonight. It’ll be better to seek for Uranus in a week or two, when you can star-hop to Uranus in a dark, moonless sky.
The opposition of Neptune comes on September 29 at 7 hours Universal Time (2:00 a.m. Central Time). However, the Earth swings closest to Uranus for the year on September 28, at 13 hours Universal Time (8:00 a.m. Central Time). Earth doesn’t come closest to Uranus precisely at opposition. That’s because the orbits of Earth and Uranus aren’t perfect circles, and the orbital planes of the two planets don’t perfectly coincide. All the same, Uranus’s closest approach to Earth always happens on or near its opposition date.
Near-infrared view of Uranus, its rings and some of its moons. Credit: European Southern Observatory
Opposition is the astronomical word used to describe a planet that’s opposite the sun in Earth’s sky. At opposition, planets rise at about sunset and set at about sunrise, a bit like a full moon. What’s really happening here? Nothing at all unusual. It’s only our own planet Earth moving in its yearly orbit around the sun. Every year, we fly between the sun and Uranus in our smaller, faster orbit.
Uranus is barely visible to the human eye even under excellent conditions. To view Uranus yourself, you will need clear, dark skies and probably a good pair of binoculars and a detailed sky chart. You would search for Uranus in the vicinity of the Great Square of Pegasus and the “Circlet” in the constellation Pisces the Fish.
Uranus was the first planet discovered since ancient times, and the first discovered by the telescope. British astronomer Sir William Herschel discovered it in 1781. All of a sudden, the radius of the known planetary system doubled in size, from approximately 10 times the Earth’s distance from the sun to about 20 times the Earth’s distance. Before the discovery of Uranus, Saturn – the 6th planet from the sun – was the farthest known solar system planet.
Interestingly, although Uranus technically can be seen by the unaided human eye and quite possibly was seen before, no one knew it was a planet. Try your luck after the moon leaves the evening sky in a week or two, but be sure to have binoculars and a sky chart handy.