ASTRONOMY ( HERCHEL)

BEING THERE OCCULTATIONS AND ECLIPSES

Mike Tabb & Dick Phillips, on 7 September 2001

Several times each month the Moon passes in front of an easily identifiable star: an occultation takes place. This phenomenon is useful for comparing Universal Time (UT - once called GMT) the length of a day as defined by the Earth's rotation relative to the mean Sun with Ephemeris Time (ET) the length of a day used in astronomical calculations - which include that of the Moon's position.

Universal Time varies slightly. There is a fairly steady slowing of the Earth's rotation lengthening the day by 2 ms (0.002 seconds) per century; regular fluctuations with periods of 10 years, seasonal and 2 weeks; and irregular fluctuations, possibly due to volcanic eruptions and unknown causes.

The steady slowing, even though it is only 2 ms/century, affects astronomical and historical timings. For example, a solar eclipse recorded by astronomers of the Han dynasty on July 17th in 187 BC occurred several hundred miles east of the path of totality calculated today. Studies of the growth rings in fossilised coral reveal that four hundred million years ago there were 400 days in a year.

Amateur astronomers can, and regularly do, observe occultations and report the results to the professional astronomers - now co-ordinated in Japan. It requires only a simple telescope, or even binoculars for some of the brighter stars; accurate knowledge of one's position (latitude and longitude); and accurate timing of the disappearance behind the Moon or re-appearance of the star, to within a second.

In the special case where just the edge of the Moon occults a star, it is possible to see a `grazing occultation'. Then the starlight flashes on and off as the star is intermittently obstructed by the mountains of the Moon . Such an occultation would be visible from the centre of Bath in December this year if only there was no artificial lighting. By going into a dark area of the countryside to the SW or NE of the City, it could be observed with a suitable telescope.

When the Moon passes in front of our local star the Sun the occultation is called an eclipse. It may be total, partial or annular, depending on the line-up of the viewpoint on Earth, the Moon and the Sun, and the distance of the Moon from the Earth, which is not constant. At a particular place on Earth an eclipse only recurs every 460 years, which is why it was so impressive in ancient times. It is fortuitous that, at present, the relative distances and sizes of the Moon and the Sun can produce a total eclipse. As the Earth and Moon gradually move relative to the Sun this will no longer occur.

Mike Tabb has travelled to see eclipses in Hawaii, India, Hungary and Zimbabwe and showed slides of the last three. In Hawaii, cloud frustrated those at the planned observing site, other people staying at their hotel had a beautiful view of the eclipse! Totality lasted only a few seconds in one case and over six minutes in another. He described a number of interesting effects on the ground shadows become distorted to match the shape of the section of the Sun still visible, and bands of shadows race across the ground before the main darkness of the eclipse arrive.

Donald Lovell