Ceres at opposition.

The first asteroid to be discovered, (1) Ceres, has recently been assigned dwarf planet status. It is the largest member of the Main Belt of asteroids, sufficiently large for its surface to collapse under the force of its own gravity resulting in a near-spherical body roughly 940km in diameter. Although Ceres travels around the Sun once every 4.60 Earth years, its orbit is slightly elliptical and inclined almost 11[degrees] to the plane of the solar system. As such it exhibits oppositions rather similar to those of the planet Mars, some occurring when the dwarf planet is near aphelion, termed an aphelic opposition, and some when the planet is near perihelion. When Ceres reached opposition on 2009 February 25, just 15 days after perihelion, the planet was 1.5832 AU from the Earth, closer than it will be for the next 1,000 years or so.

In anticipation of this rare event, BAA member John Sussenbach took webcam-type images of Ceres during a period of good seeing on 2009 February 14. The montage depicting the dwarf planet alongside an image of a star was obtained using a 28cm Schmidt-Cassegrain telescope from his home in Houten, The Netherlands. Technical details were:

   Atmospheric conditions: Seeing 7/10, Transparency 8/10 Telescope:
   Celestron C11 with Televue x3 Barlow, Astronomik RGB filters
   Camera: DMK 21AF4.AS monochrome planetary camera Exposure time: 1/5
   sec; gain 100%; gamma 40%; per colour 2x2 min exps. Total number of
   frames: 980 per colour Processing: Registax 4.0 stacking the 500
   best frames per colour

John's images clearly show Ceres as a tiny planet rather than a conventional 'asteroid': a term originally coined by Sir William Herschel during the early 19th century to describe the 'star-like' appearance of such objects when seen visually in a telescope, in contrast to the usual appearance of a planet or comet. Helped somewhat by the record close approach to the Earth, the angular diameter subtended by the dwarf planet at the time of John's observation was about 0.83 arcseconds. The limb darkening and spherical shape of Ceres shown in the stacked colour image is clearly evident when compared to the seeing disk of the star, Denebola (Beta Leonis) recorded at the same time under similar conditions. This is the first time I have seen the disk of Ceres imaged and successfully resolved by an amateur observer--an excellent achievement.

[ILLUSTRATION OMITTED]

Although Ceres was particularly close at the time of John's observations, that did not mean the planet was at its brightest for 1,000 years. The brightness of an airless body such as Ceres depends on the way light is reflected and scattered from its surface, and that depends on the phase angle the object reaches relative to the position of the Sun as seen from the Earth. If such an object has a very low phase angle then multiple reflections can occur at the surface, enhancing its brightness by as much as 30%.

[ILLUSTRATION OMITTED]

The table provides details of close approaches to the Earth at the near-perihelic oppositions of 2009, 2012, 2018, 2023 and 2027. The data were derived from the JPL Horizons web-interface.

Over the next few decades, Ceres will be brightest on 2012 December 18 when it will be 0.15 magnitudes (i.e. 15%) brighter than at the time of this year's close approach and may very well be visible to the naked eye when seen in a very dark sky.

I would like to express my sincere thanks to both John Sussenbach and Jean Meeus for their help in preparing this note.

Richard Miles, Director, Asteroids and Remote Planets Section

Close approaches of (1) Ceres

                                        Phase
Date (UT)         [Delta]    R          angle         V
                  AU         AU         ([degrees])   magnitude

2009 Feb 26.0     1.58320    2.54737    6.43          6.88
2012 Dec 18.2     1.68495    2.66850    0.69          6.73
2012 Dec 20.4     1.68428    2.66704    1.21          6.78
2018 Feb 01.6     1.60186    2.57040    5.12          6.86
2023 Mar 21.1     1.59904    2.56830    6.48          6.92
2027 Jan 09.5     1.62631    2.60360    3.05          6.79