Occultation of Saturn will occur on March 21

Press, 29 February 1984, Page 24

Occultation of Saturn will occur on March 21

Early risers will have the opportunity on March 21 of seeing the ringed planet Saturn disappear behind the bright limb of the Moon at 5.52 a.m. The occultation will occur against the bright dawn sky so some instrumental assistance will be required to view this unusual event. Viewers should watch the planet and Moon steadily approaching each other for several minutes before the time of disappearance. The plant will reappear from occultation on the dark side of the Moon at 6.28 a.m. The Sun will then be rising so a telescope will be necessary to see this phase. The planet, Uranus, will also be occulted two mornings later at 6.57 a.m. on March 23. This event will occur in daylight and will be difficult to see even with a telescope. The present observations of comet Crommelin as test for observing and communications by the International Halley Watch, recalls the work of New Zealand’s first comet hunter. He was John Grigg, who had migrated to this country in 1863. He settled in Auckland but five years after his arrival he moved to Thames, attracted by the discovery of gold there the previous year.

Grigg built an observatory in Thames. His main telescope was very small by modern standards and was a refractor of just under 9cm aperture. He also had a small transit intrument which he used each day to determine the correct time. This enabled him to adjust the large clock in his office below his observatory. This was the clock from which the citizens of Thames got the correct time.

Grigg had an all absorbing interest in astronomy and observed anything of interest, but he is remembered for his work on comets. He discovered his

first comet on July 22, 1902. This was a rather faint object and, when discovered, was between the constellations of Leo and Virgo. Actually Grigg was the only person to observe this comet. This was because the slow communications of that time meant that news of his discovery did not reach other astronomers until after the comet had faded. The remarkable part of this discovery was that Grigg not only discovered the comet but he made sufficient accurate observations to enable him to also compute its orbit. This orbit was so accurate that when the comet was rediscovered by Skjellerup at its 1922 return it was obvious that the two objects were different returns of the same comet. It is now known by their joint names as Comet Grigg-Skjellerup. On April 17, 1903, Grigg discovered his second comet. This was a very faint object in Eridanus and had already passed perihelion. This comet is Comet Grigg, or 1903 111. It was almost seven years before Grigg found his third comet. On April 8, 1907, he discovered a seventh magnitude object in the constellation, Caelum. This comet at that time had no tail and the discovery was made under very poor sky conditions. The comet was independently discovered by Mellish at the Washburn Observatory in the United States six days after Grigg. This comet is now known as Comet Grigg-Mellish, or 1907 11. Actually the famous American astronomer, Barnard, found the comet on a plate he had taken at the Lick Observatory on April 13 but by the time he examined his plate its discovery was already known.

The name of John Grigg is also associated with the short-period comet Encke.

On June 7, 1898, Grigg was the first person to see this comet on its twenty-eighth recorded return. It was then only three degrees above the horizon. Grigg located the comet from his own calculations of its orbit. Four days after Grigg’s observation the comet was found by Tebbutt in Australia. Naturally Grigg made many observations of the spectacular appearance of Comet Halley in 1910. He and Father Kennedy, of Greenmeadows, were the only people in New Zealand to take successful photographs of this comet. This brings out another facet of Grigg’s activities. The cameras he used were made by himself. He also made the camera that was used to photograph the 1882 transit of Venus. He was also skilled at making mountings and clock drives for telescopes. He supplied the clock drive for the expedition to New Zealand for the Venus transit in 1882. In searching for comets Grigg preferred to search in the morning sky. His methods were to start his search at 2 a.m. and to continue to about 5 a.m., or in winter somewhat later until dawn came. He worked over the evening sky only when he was looking for a comet that had been predicted to return. He apparently adopted the method of sweeping across a wide arc of the sky. Then moving his instrument half its field diameter he swept back in the opposite direction. This procedure he continued until he had searched a large section of the sky. He came to know the nebulae and clusters, which in his small telescope, could easily be mistaken for a comet, but he would always check and never trusted purely to his memory of where these objects were. Grigg became very well known in Thames, not only

as the only source for accurate time and information of events in the sky, but for his mechanical ability and as a lecturer. He established a meridian mark on Una Hill. This was close to the Una Mine, the workings of which he thought might damage his meridian mark, so he set up another mark further from the mine. He thus established an accurate position for the township of Thames. Grigg has been followed by quite a number of New Zelanders who have discovered comets, but he was the pioneer who worked largely alone without the benefit of the rapid communications of today and without the superior instruments now in use. Mercury reaches superior conjunction on March 8 and then passes into the evening sky. It will be setting soon after the Sun at the end of the month and will not be favourably placed for viewing. Venus will be a brilliant object in the morning sky, especially during the first fortnight of March. It then begins to draw closer to the Sun, but will still be visible for a short time before dawn. Mars, in the constallation of Libra, continues to brighten and by the end of the month will have a magnitude of -0.5, making it brighter than Saturn by one magnitude. Mars is now in a better viewing position, rising well before midnight. It will outshine all the surrounding stars and can easily be picked out by its reddish colour. Jupiter will be rising about an hour before midnight by the end of the month. It is in Sagittarius. Saturn has been close to Mars in the past month and the two planets are now moving apart. Saturn is also in a good position for viewing rising well before midnight.