Astronomy and Parallax

  Talk given by Martin Frey Wednesday 20th October 2004

Martin Frey is a founder member of the Tenterden Astronomical Society and as part of his Basic Astronomy talks gave us an intriguing talk on Parallax in Astronomy.  He presented his talk using PowerPoint on his laptop and displaying the images through a digital projector with great effect.

Looking through some of his old family photographs Martin found some taken when he was 12 years old and was struck by how many pictures of his knees existed and realised that this was the result of parallax when his mother used the viewfinder lens, which was separate from the camera lens.

He compared this with a recent experience he had had whilst observing the occultation of the planet Saturn by the moon in April 2002 from Appledore in Kent.  At the time of occultation a friend of his in Newcastle rang to say he had already seen it.  Worse was to come when Martin missed the reappearance of Saturn!  The reason was because of parallax due to their relative positions and the closeness of the moon to the earth.  From Newcastle the occultation took 38 minutes, but from Appledore the occultation took place nearer the rim of the moon and took only 25 minutes.

Martin briefly talked about the history of using the Transits of Venus to estimate the distance of the Sun from the Earth.  In 1639 the Reverend Jeremiah Horrocks was the first astronomer to use the timings of the transit of Venus to calculate and estimate of the distance of the Sun from the earth.  He made the distance 59,400,000 miles, 64% of the accepted distance today.

In 1761 and 1769 further observations of the transits of Venus enabled astronomers such as Jean-Baptiste, Chappe d'Oloroche in France and David Rittenhause in Pennsylvania to achieve much better results.  With more accurate methods of measurement then available, estimates of between 91,000,000 miles and 97,500,000 miles were made.  (97 to 104% of the distance known today)

We were also told an interesting story about measurements taken by Aristarchus of Samos who lived in 320 to 250 BC.  He waited until the earth/moon/Sun formed a right angle, then by measuring the angle between the moon and the Sun he produced a predicted distance 390 times the distance of the earth to the moon.  But found this quite unacceptable and announced a much lower distance.

Martin referred to an article in the September issue of Sky and Telescope this year, called, "The lunar Parallax Demonstration Project" written by Peter Lawrence, a friend of his.  In it Peter describes a project where he put together a number of photographs of the moon taken at various sites around the world at a precise time during the lunar eclipse of 9th November 2003 when the contrast between the background stars and the luminosity of the moon was at its best.  He found the effects of parallax quite easy to see.  He also noted photographs taken in the USA and Europe showed slightly different parts of the lunar surface, another effect caused by parallax.

Martin also noted that at any one time from one point of observation on the earth's surface, a maximum of 50% of the surface of the moon can be seen, but because the orbit of the moon is elliptical, this enables us during a period of time to see a total of 59% of the surface.  This effect is called libration.

It was Copernicus who openly stated that the Sun was at the centre of the solar system with the stars at a vast distance.

James Bradley in the 18th century noted the displacement of stars in a year, and from this confirmed the earth's orbit around the sun although the angular distance was too fine to be measured at the time.

Bessel later used Bradley's star charts and with more accurate instruments improved them to produce the most accurate star charts yet.  In 1838 using parallax he observed 61 Cygni moving against the background stars and estimated its distance to be 10.3 light years away.  (Now known to be 11.5 light years distance, showing just how careful he must have been with his measurements)

In fact Martin said that Thomas Henderson, Royal Astronomer at the Cape of Good Hope, used parallax to measure the distance of Alpha Centauri as 4.3 light years; five years earlier than Bessel had published his measurements, but Henderson didn't announce his results for another two years later.

The talk closed with Martin saying that some star positions are now so accurately know that they can be "visualised" from a theoretical point beyond them and looking back towards the solar system.

The position of stars will be even more accurately know during the GAIA mission which is due to take place in about 2010, and then the story will be continued...


Our next meeting is on Wednesday the 17th of November when the speaker will be our own member Ian King whose talk is called "Wide Field Imaging".

As usual, the meeting will be held in the Drama Studio at Uplands College.  The doors open at 7.15 and the meeting starts at 7.30 prompt.

There will also be the opportunity to discuss informally the future of the Society in advance of the AGM in December.  We have now received a number of offers from members prepared to join the Committee, which is great news, although it would be good to hear from one or two more members.



On Wednesday December the 15th we hold the Society's Annual General Meeting and we hope as many members as possible will be able to come along.

Our speaker at this meeting will be Norman Walker who was a professional astronomer and has visited us before.  The title on this occasion will be "What you always wanted to know about astronomy and were afraid to ask".  He is very good and it is worth thinking of those astronomy related questions whose answers have eluded us in the past.


On a planetary note

I know that the clocks have now gone back and the mornings are that little bit lighter for a while, but it is worth grabbing a pair of binoculars just before dawn on a clear morning to observe Venus, Mars and Jupiter.  They are beginning to separate a bit now but still make a good group and although the moon is about to reappear it will be a thin crescent for a while.



It is suggested that one if not two of the rules included in our Constitution are in need of amendment.  Next month's AGM would be a good time to take action.  One can only assume that in December 2001 no ordinary member was burning/stimulated/inclined to take their turn on the Committee of the Society.  In order to keep it going in the face of such enthusiasm, the majority chose to weaken the draft rules before their adoption.  It is therefore proposed that paragraph 5.5 should be MODIFIED, this time to read, "The elected term shall be for ONE year".  In addition a draft rule deserves to be REVIVED as paragraph 5.8, this time to read "No member shall hold the same office for more than THREE years".  A committee no longer locked in to a job for life is more likely to take its brief opportunity to promote new ideas.

Now might be the time to consider a reduction in the size of the Committee from the nine listed in December 2001 to seven, namely: Chairman, Secretary, Treasurer, Editor, Publicity & Web Site, Director of Observations, Member without-portfolio.  All these matters can be discussed at the November meeting with a view to recommendations being voted on and adopted at the AGM to be held on the 15th December 2004.

As you know part of the 2003/4 Committee will be standing down in December.  The other part is willing to provide a measure of continuity.  It is encouraging to learn that at least three ordinary members have indicated their willingness to join the new committee.  More are invited to come forward so that we can HOLD AN ELECTION on truly (non political) democratic lines.

You will be pleased to learn that we did conclude the last financial year with a modest surplus of £80.  If nothing else the old team will be leaving a comfortable balance at the bank to their successors for the future benefit of the Society.  Just a gentle reminder - Subscriptions are now due for our year, which began on 1 November 2004.

It is worth remembering that from a standing start 7 years ago the Society has become (by F.A.S. standards) one of the larger middle size societies.

Ian Reeves

A number of copies of the Society's Constitution will be available at our November meeting.  Ed.


Editor:           Geoff Rathbone          01959 524727


Any material for inclusion in the December Newsletter should be with the Editor by November 30th 2004