WADHURST ASTRONOMICAL SOCIETY
Alan Drummond of the
Crawley Astronomical society came to our June meeting
to give us a talk on 'Eclipses', primarily eclipses
of the sun.
We heard that eclipses of the sun are more frequent than most people realise. Some form of solar eclipse occurs once every six new moons and that annular eclipses like the one observed on the 31st of May this year is twice as common as total eclipses.
The sun has a very strong effect on the orbit of the moon and because of this the moon never follows exactly the same path. The orbit of the moon is more complex than one would have thought.
It's possible to work out the maximum amount of time for an eclipse to last.
The moon and its' shadow moves at 3400km/hr eastwards
The earth at the equator rotates at 1700 km/hr eastwards
The minimum diameter of the Umbra (darkest part of the shadow cast by the moon) on the surface of the earth is 220 km.
220 x 60 = 7¾ minutes is the maximum duration.
We learned that there are different ways of describing a month and using these different descriptions we are able to predict eclipses.
Types of month
1. New moon to new moon is called a lunation month it has a duration of 29.53059 days, this period is very constant and very useful for eclipse predictions.
2.Node to Node (this is where the moons' orbital path crosses the ecliptic) is known as a Nodical month this lasts 27.21222 days.
3. Perigee to Perigee ( the closest position of the moon to the earth) this is known as the Anomalistic month and lasts 27.55455 days.
The node shifts 30.67 degrees each month and in six months it has shifted just over 180 degrees, that is why it is likely that a type of eclipse is going to happen once in every six months. It is however possible for an eclipse to occur once in five months.
There are patterns of eclipses. The circumstances repeat themselves once every Saros Period (18.03 years).
Fifty years ago a Dutch astronomer called Van Den Bergh worked out that just by using the information from one eclipse you can predict 100's of future eclipses. He determined a period of 28.95 years that he called the Inex.
Alan Drummond concluded his talk with a series of photographs that he had taken of the recent eclipse in May, the 1999 eclipse and an eclipse he had witnessed in New Mexico in the early nineties where the event happened almost directly overhead in a cloudless sky.
The Chair expressed his gratitude on behalf of the WAS for a very in depth lecture on the celestial mechanics of eclipses!
Finally the Chair reminded the society of the forth coming Fete at Speldhurst on the 28th of June. It is recommended that any helpers should make their way to the WAS pitch in good time to avoid the chaos of vehicle access. Many helpers will be arriving between 10.30 and 11.30 to help set up telescopes and the small display. Any one interested in participating should contact the Chair for an 'Exhibitor's Pass'. Lets hope the weather will be kind for us again this year!
The next meeting of the society will be on Wednesday July 16 2003. As usual, the doors open at 7.15 and the meeting starts at 7.30 prompt. Unfortunately, the editor is not sure who the speaker is for this evening, but has been informed that the subject is camcorders!
The observing sessions will not take place for the next few months but should return again later in the year.
annual WAS Barbecue is to be held on Saturday August
30. The venue is Greenman Farm, Wadhurst, as per usual
and the event will start at around 7 pm. Please bring
your own food (to cook!) and drink, all cooking facilities
will be provided as usual.
All members and their families are most welcome to come to the event and if you wish to bring your telescope even better! Please direct any queries to Michael Harte on 01892 783292. Thanks once again to Michael and his wife for acting as our hosts for the evening.
For those members who are new to the society Greenman Farm is on the B2099 Wadhurst to Frant road. From Wadhurst come past the railway station, under the railway bridge, and Greenman Farm is on the left. There is a map with the area indicated available here.
Chairman: Murray R. Barber 01892 654618 email@example.com
Secretary: Ian King 01892 836288
Treasurer: Ian Reeves 01892 784255
Editor: Peter Bamblett 01732 368656 firstname.lastname@example.org
Publicity & Web Site: Michael Harte 01892 783292 email@example.com
Dir. of Obs.: Sean Tampsett 01892 667092 firstname.lastname@example.org
Librarian: Joan Grace 01892 783721
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