Astronomy

Press, 9 January 1981, Page 7

Astronomy

By

F. M. BATESON

Largest asteroid at opposition

Ceres, the largest asteroid, comes to opposition on January 11. It will then be in Gemini lying almost half-way between the bright stars Castor and Pllux. In spite of its position well north of the celestial equator this will be a good opportunity for the casual viewer to locate an asteroid.

At opposition Ceres will have a visual magnitude of 7.3, making it an easy object to find with binoculars. It will appear as a star-like point that can only be separated from the surrounding stars by its motion against the stellar background. - On January 6 Ceres can be found by extending a straight line from Pollux to the fourth magnitude star, Sigma Geminorum, which lies close to Pollux and in the direction of Castor. Ceres will be the same distance beyond Sigma that this star is from Pollux.

By January 11 Ceres will have moved to half way between Pollux and Castor, and on January 26 it will lie to the west of Castor and just beyond the fourth magnitude star, Rho Geminorum. Ceres has a diameter of 1000 km. It was found on January 1, 1801, and was the first asteroid to be discovered.

It belongs to the group of asteroids termed carbonaceous. Such objects are thought to be rich in carbon on their surfaces and are therefore dark.

By measuring the amount of incident sunlight reflected it has been found that asteroids appear to fall into two distinct types. There are those, like Ceres, that are very dark while others are very- much better reflectors so that it is believed that their surfaces are composed of silicate materials.

In this respect their surfaces resemble those of the inner, or terrestrial, planets. Current theories favour the idea that the difference in surface composition is due to the difference in distance from the Sun at which the two groups were formed. This theory’ suggests that the asteroids condensed out of the original solar nebula in much their present positions., - It is considered that different materials would have condensed from the gas cloud at different distances from the Sun. The theory is supported by the

fact that most of the asteroids with silicate surfaces lie closer to the Sun than those that appear carbonaceous. Asteroids have been given a wide variety of names. At first they were generally given a female name taken from mythology. The available names from this source soon ran out with the result that the discoverers, who have the privilege of suggesting names for their discoveries, honoured their country, ci.y, their predecessors or merely a person who was dear to them. Always the female version of the name was

used, such as Hispania, Chicago, Galilea and Dolores.

A discoverer normally may not name an asteroid until its orbit has been calculated and it has been again observed after completing at least one circuit of the Sun. Normally they receive a number, which is a running index of the order of discovery. Thus Ceres is 1, while Eros is numbered 433. To confuse matters still further there is also a system of using the year of discovery and a system of two letters pending the determination of the orbit. While female names are generally given, masculine names are bestowed on unusual asteroids.

The giant planet Jupiter exerts a powerful influence on the orbits of asteroids. Gaps occur in the asteroid belt due to perturbations by Jupiter. An asteroid lying at about five-eighths Jupiter’s distance from the Sun would have a period exactly half that of Jupiter. This would result in the asteroid being close to Jupiter on every second circuit of the Sun. The repeated attraction by Jupiter would eventually force the asteroid out of its orbit leaving a gap. There are a number of these gaps, known as Kirkwood Gaps after the astronomer who was the first to explain them. There are, however, two points in the orbit of Jupiter where asteroids can remain indefinitely. These two points, which, with Jupiter and the Sun, make equilateral triangles. The positions are exactly sixty degrees ..ahead ■ of, or behind. Jupiter. .- At these points aste-

roids circle the Sun in almost the same period as Jupiter being one sixth of a circuit ahead of or behind the giant planet. Their motions are complicated as they oscillate back and forth around the point of stability. These asteroids are known as the Trojans, being named for the Homeric heroes. It is the practice today to name those east of Jupiter after the Greek warriors and those to the west after the Trojans. This convention came into use after the first three had been discovered with the result that there is one Greek

“spy” amongst the Trojans and one Trojan “spy” amidst the Greeks. In 1931 there will be four eclipses, two each of the Sun and Moon.

The first is a penumbral eclipse of the Moon on January 20. At such times the Moon does not pass into the Earth’s umbra, or real shadow. As a result on January 20 the Moon only passes into the penumbra, or region in which some sunlight is present. As a result all that can be noticed is a faint dimming of that portion of the Moon that is in the penumbra. The first solar eclipse in 1981 will be an Annular Eclipse, visible as a partial eclipse over New Zealand. This event will be the subject of. next month’s notes. It will be followed by a partial lunar eclipse of July 17, the end of which will be visible from our latitudes. Three will also be a Total Solar Eclipse on July 31, which will no* be visible from New Zealand.

In January, the main inter*", in the bright planets will centre on Jupiter and Saturn, close together in Virgo. Both planets rise about 1 a.m. on January 1 and two hours earlier on January 31. They will be very close together when they rise on January 14. Saturn will be the fainter of the two planets, and will then be only a fraction of a degree "from the well known double star, Gamma Virginis. Venus may be glimpsed very low in the east in the dawn sky. Mars and Mercury will be difficult to see as they are only visible very low in the west during twilight