Astronomy Essentials

Ceres at opposition, coming up November 27

Ceres at opposition: Bright spots near center of rocky round ball.
Ceres will be at opposition on November 27. Now begins the best time in 2021 to observe it. Here’s a Dawn spacecraft view of Ceres in false color. Those bright spots on the dwarf planet’s surface caused a stir when Dawn first spied them on approach to Ceres in 2015. Image via NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA.

Ceres was the first asteroid discovered, in 1801. The International Astronomical Union reclassified it and Pluto as dwarf planets in 2006. Ceres is the largest object in the asteroid belt between Mars and Jupiter. It’s the only world in the asteroid belt with enough mass – and therefore enough self-gravity – to pull itself into the shape of a ball. So Ceres is the only dwarf planet in the asteroid belt, or even inside the orbit of Neptune, for that matter. In 2021, Ceres will be at opposition – opposite the sun in our sky – on November 27. At that time, it’ll rise at sunset and set at sunrise. It’s coming up later at night now (mid-September), but will be rising earlier. Now is the beginning of the best time in 2021 to view Ceres.

Ceres is about 600 miles (1,000 km) across, or about 1/4 the size of our moon. With its size wedged somewhere between an asteroid and a planet, Ceres was a target for study by the Dawn spacecraft.

Dawn arrived at Ceres in 2015. The spacecraft caused a stir while approaching Ceres, when it began to capture images of bright spots on the dwarf planet’s surface. People joked that the spots looked like alien headlights. But they turned out to be salt deposits from salty water inside the planet. Dawn also found a 2.5-mile (4,000-meter) high mountain called Ahuna Mons.

If you have a telescope or good binoculars, now is the time to start watching Ceres. Note that the name asteroid means starlike. From Earth, Ceres looks like a star. But because it’s so close to us, it can be seen to move in front of the stars from night to night.

Star chart showing labeled stars and small dot with cirle around it.
View larger. | Throughout September, October and November 2021, Ceres (inside yellow circle) will be meandering through Taurus the Bull. Taurus ascends in the east in the middle of the night in mid-September. But it’ll rise earlier each evening. Ceres will cut across the V-shaped Hyades star cluster in Taurus before its opposition on November 27, 2021. Chart via Dominic Ford/ TheSkyLive. Go to this page at TheSkyLive to view a more detailed star chart showing a telescopic view of Ceres.

How to view Ceres at opposition

So dust off your binoculars or telescope and head to a dark-sky site to see Ceres. The brightness of astronomical objects is measured in something called magnitude, with lower numbers indicating brighter objects. From a location free of light pollution, you can see objects down to about magnitude 6. Right now (September), Ceres shines around magnitude 8. But Ceres at its brightest in 2021 (late November) will shine around magnitude 7. So you can see you’ll need optical aid to bag this unique object. Ceres is brightening as it nears opposition!

Ceres is now moving in front of the constellation Taurus the Bull. It’ll remain in front of Taurus between now (mid-September 2021) and the time of its opposition (November 27, 2021). In fact, Ceres will spend much of fall 2021 cutting across the V-shaped Hyades star cluster in Taurus. It’ll be near the bright reddish-orange star Aldebaran, brightest star in Taurus.

The easiest time to find Ceres in 2021 might be the first few days of November. Magnitude 7.5 Ceres will pass less than a moon’s width from Aldebaran in Taurus around these evenings. If you focus binoculars or a telescope on Aldebaran, the point of light close to the star will be Ceres. Check out the finder maps below.

Star chart with 3 positions of Ceres relative to Aldebaran.
Throughout fall, Ceres has been meandering through Taurus, cutting across the Hyades cluster before its opposition on November 27, 2021. Position data via CyberSky.
Star chart with path of Ceres in green, its position at various dates labeled.
View larger. | The white dot marking the star by the label “05 Nov” is Aldebaran, the brightest star in Taurus. Ceres will swing close to Aldebaran for the first several days of November, a great time for you to try to spot the dwarf planet. Image via Dominic Ford.

Why Ceres is so dim

Ceres orbits our sun at a greater distance than Earth. Its average distance from the sun is about 2.77 times that of the Earth. And so Ceres’ brightness doesn’t vary much throughout the year. Being small, far away and dim, it requires at least binoculars to spot and even then it only appears as a point of light like a distant star.

Ceres does us no favors in terms of its reflectivity. Objects have a measurement called albedo, which is a number between 0 and 1 for how black or white they are. Very reflective fresh snow or ice can have an albedo of 0.8 or 0.9. Our neighboring planet Venus is often said to appear bright to us because its thick cloud cover reflects so much sunlight. Venus has an albedo of 0.65. On the other hand, low-albedo objects absorb most sunlight and are quite dark. Charcoal and fresh asphalt both score 0.04 for their albedo. Ceres’ albedo is 0.07. It’s practically hiding in the dark against the blackness of space.

Ceres may be the brightest dwarf planet, but only because it lies within the asteroid belt, the zone of solar system debris between Mars and Jupiter. Pluto’s albedo is 0.30 and Eris’ is 0.86, one of the highest albedos in the solar system. Ceres is only about three times farther from the sun than Earth. Compare that to Pluto, which is 40 times farther from the sun than Earth. And Eris is a whopping 68 times farther from the sun.

Top view of solar system with sun, orbits of inner planets and of Ceres.
View larger. | Ceres at opposition means that it is opposite the sun in our sky. Earth is passing between Ceres and the sun. This view shows where planetary bodies will be in our solar system on November 27, 2021. The yellow ball at right is the sun. Image via Dominic Ford/

Bottom line: With Ceres at opposition November 27, the dwarf planet is closest to Earth and therefore brightest, making it a great time to observe. Ceres will be near Aldebaran around the first few days of November.

September 20, 2021
Astronomy Essentials

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