Partner Series
Dwarf Planet Ceres Reaches Opposition in Night Sky
This star chart shows the location of the dwarf planet Ceres just before 1 am local time on Wednesday, July 29, 2015, as seen from mid-northern latitudes.
Credit: Starry Night Software

With all the news about Pluto this month, most skywatchers may not be aware that Ceres, the brightest and nearest of the dwarf planets, reached its opposition to the sun on Saturday (July 25).

An object is said to be in opposition when it is directly opposite the sun in Earth's sky. When an object is in opposition, it rises at sunset, is visible all night and then sets at sunrise.

When Ceres was discovered by Giuseppe Piazzi on the first day of the 19th century, Jan. 1, 1801, it was initially thought to be a planet. Other objects were soon discovered in the gap between the orbits of Mars and Jupiter, so a new name was proposed for these tiny bodies, "asteroids." This was because of their resemblance to stars in the telescopes of the day. [Amazing Photos of Ceres]

When the International Astronomical Union proposed a new category, "dwarf planet," in 2006, Ceres was included, along with Pluto, Eris, Haumea, and Makemake. So tiny Ceres has been called three different things in the 214 years since it was discovered: planet, asteroid, and now dwarf planet.

Despite its recent designation as a dwarf planet, Ceres is still considered to be an asteroid by most astronomers. As such, it is the largest of the asteroids at 592 miles (952 kilometers) in diameter, almost twice the diameter and four times the mass of the next largest asteroids, Pallas and Vesta. On the other hand, Ceres is far smaller than any of the other designated dwarf planets. It is also smaller than 15 of the moons in the solar system, including Earth's moon.


Because of its faintness, magnitude 7.5 at opposition, detailed charts are needed to distinguish Ceres from the stars it's passing in front of. Our chart shows its position on the morning of opposition, Saturday, July 25. If you try to find it on any other night, you will need to plot its position with a program like Starry Night or SkySafari.


On March 6 this year, the Dawn spacecraft entered orbit around Ceres, and has since been returning detailed images of Ceres' surface. Thus, 2015 marks a special year for the exploration of the solar system, with Dawn orbiting Ceres and New Horizons flying close to Pluto.
