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.
Therefore;-
220
x 60 = 7¾ minutes is the maximum duration.
3400-1700
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!
Valerie Barber
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.
The
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 murray.barber@virgin.net
Secretary: Ian King 01892 836288
Treasurer: Ian Reeves 01892 784255
Editor: Peter Bamblett 01732 368656 pbamblett@hotmail.com
Publicity & Web Site: Michael Harte 01892 783292 michael@greenman.demon.co.uk
Dir. of Obs.: Sean Tampsett 01892 667092 sean_tampsett@hotmail.com
Librarian: Joan Grace 01892 783721