Ceres (dwarf planet)
Observation

When in opposition near its perihelion, Ceres can reach an apparent magnitude of +6.7. This is too dim to be visible to the average naked eye, but under ideal viewing conditions, keen eyes may be able to see it. Vesta is the only other asteroid that can regularly reach a similarly bright magnitude, while Pallas and 7 Iris do so only when both in opposition and near perihelion. When in conjunction, Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars; thus, it can be seen with such binoculars in a naturally dark and clear night sky around new moon.

An occultation of the star BD+8°471 by Ceres was observed on 13 November 1984 in Mexico, Florida and across the Caribbean, allowing better measurements of its size, shape and albedo. On 25 June 1995, Hubble obtained ultraviolet images of Ceres with 50 km (30 mi) resolution. In 2002, the Keck Observatory obtained infrared images with 30 km (20 mi) resolution using adaptive optics.

Before the Dawn mission, only a few surface features had been unambiguously detected on Ceres. High-resolution ultraviolet Hubble images in 1995 showed a dark spot on its surface, which was nicknamed "Piazzi" in honour of the discoverer of Ceres. It was thought to be a crater. Visible-light images of a full rotation taken by Hubble in 2003 and 2004 showed eleven recognisable surface features, the natures of which were undetermined. One of them corresponded to the Piazzi feature. Near-infrared images over a whole rotation, taken with adaptive optics by the Keck Observatory in 2012, showed bright and dark features moving with Ceres's rotation. Two dark features were circular and were presumed to be craters; one was observed to have a bright central region, and the other was identified as the Piazzi feature. Dawn eventually revealed Piazzi to be a dark region in the middle of Vendimia Planitia, close to the crater Dantu, and the other dark feature to be within Hanami Planitia and close to Occator Crater.
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