Bits and Bobs

Talk given by Greg Smye-Rumsby at the April meeting

Greg Smye-Rumsby is a member of the Orpington Astronomical Society and also presents illustrated talks at the Greenwich Planetarium.  He is also well known to readers of Astronomy Now as the resident technical illustrator and many of his skills were put to use in his talk.

Greg began his talk by saying that he liked to call his talk "Bits and Bobs" so that he could decide at the very last moment what the subject should be.  In this case he was going to talk about the moon.

The moon is about a quarter the size of the Earth and it would be possible to lay 29 earths in a line between the moon and us.

To give an idea of dimensions, Greg held a globe in his hands saying that if this was the earth (about 20 centimetres in diameter) then the International Space Station is orbiting inside his fingers because its orbit is so shallow.

We then looked at drawings made by Galileo who was one of the few people to have only seen rocks whilst others claimed to have had seen trees and creatures.

The moon is mainly made up of craters, mountains and seas.  The seas are the results of ancient volcanic activity, with the craters being formed by impacts over billions of years.  Greg said that virtually all lunar craters are circular suggesting that they were the formed by vertical impacts.

In fact experiments show that impact angles of less than 4 degrees where the only ones to produce elongated craters and there were very few of them to be found.

Just south of Mare Crisium in Mare Fecunditatis are two such craters called the Messier Twins.  The craters are elongated, and leading to them is a pair of long streaks across the surface of the moon looking as though whatever struck the moon had scraped the surface before impact.

Patrick Moore believes that there is still some evidence of active lava flows.

Using an imaginary large sack of Lego poured out over the floor; Greg invited us to imagine how the pile would spread out over the floor in an attempt to simulate the creation of the Solar System.

At the centre of the pile, there is enough material to build a sphere (the Sun) but further out, the pile thins and there are now not enough bricks to form more than planets.

An interesting explanation of why the inner planets are not gas planets like those further out such as Jupiter, Saturn, Uranus and Neptune is because any gas that had surrounded the inner planets was drawn off by the Sun's incredible gravity.

It is now thought that, in its early stage the Earth was struck by an object about half its size, which vaporised rock.  From the debris, the Earth reformed but now had a moon.  This moon had a plastic core with the result that there was no magnetic field.  Also, the moon never had any water.

After time the moon settled and just presented one face to the Earth.

The moon's orbit around the Earth is slightly elliptical and tilts by about 5.1 degrees from the Earth's own orbit around the Sun.  This means that as the moon orbits the Earth, it is possible to see slightly more of the moon's face than if the moon's orbit had been a perfect circle.  This is known as the moon's libration.  It means that we can actually see 59% of its surface despite being in a synchronous orbit.

One interesting fact Greg pointed out was that over the next few months the orbit of the moon takes it through M44, a cluster in the constellation of Cancer, creating a number of lunar occultations.  (More of occultations at our May meeting!)

We were shown images of the Zimbabwe solar eclipse taken in 2001 where, by adjusting the exposure, it was possible to make out lunar surface details lit by Earth Shine.  It was also pointed out that solar eclipses were only visible through the incredible coincidence of the angular size of both the Sun and the moon are almost exactly the same.

We saw a slide of the phenomenon known as Bailey's Beads caused by the Sun appearing between the mountains on the moon just as the Sun becomes visible after total eclipse.

Greg concluded his talk by referring to a couple of illusions.    The apparent subjective size of the moon when close to the horizon compared with when it is high in the night sky near the zenith where the human eye has no earthly reference makes the brain over compensate, making the moon appear much larger when low down.

The other illusion involves astronauts on the surface of the moon where there is no atmosphere so there is no atmospheric mist.  The result is that mountains look as though they are a couple of kilometres distant while in fact they are more likely to be at least 25 kilometres away.

The lunar surface is made up of microscopic clinkers that are extremely sharp, not having been eroded through wind and movement.  Once this material has stuck to the surface of the astronaut's suits, it can cause damage through sharp scuffing.  To prevent this in future missions, the suits will be held outside the space-lander and are then entered through access doors from inside the vehicle.  The suits are then sealed at the back and released from the vehicle once the astronaut is inside.   The reverse action is then used when leaving the suit so that at no time will the outside of the suit bring the erosive dust inside the vehicle.


Wednesday 21st May 2008 Our own Brian Mills, who contributes the excellent Sky Notes each month, is giving a talk about "Occultations".  For some time Brian has observed occultations and helped to take accurate time measurements of the events.  These measurements are then coordinated at an international centre.

The meeting begins at 1930 although members are invited to arrive anytime after 1900.  This is a good time to exchange ideas and discuss problems.

The venue as always is in the Upper Room of the Methodist Church at the east end of Wadhurst Lower High Street, opposite Uplands College.  (For those with SatNav -  the  Post code is TN5 6AX)


During June:  A trip is being arranged to visit the Great Transit Circle at Greenwich Observatory.  This will be either Friday 13th or 20th of June.

Wednesday 18th June 2008 Because this is one of the shortest nights of the year, in recent years the Society has held a members evening when we can bring telescopes, binoculars and other aids to amateur astronomy and chat about their use and discuss problems.  There will also be a short video on an astronomical subject.  More information nearer our June meeting.

Wednesday 16th July 2008.  There will be a talk given by James Fradgley called "Orbital Oddities - Strange Goings-on with 3 or more bodies" covering Lagrange Points, Resonances, Roche Limits, and lots of odds and ends with simulations. James is a member of the Bournemouth Natural Science Society in Dorset.




The Royal Observatory Greenwich has not yet confirmed the June date for our visit.  Almost certainly it will be Friday 20th of June, although the 13th still remains our back-up date.  We hope to know by our May meeting.

At present the 16 members who have shown an interest in going are:

Michael Berks        Phil  Berry        Rose Bond        John  Daw       Joan Grace
Douglas  Hall          Michael  Harte Geoff  Lezemore Mrs. Lezemore
Angus  Macdonald   Brian  Mills   Gavin Mills    Larry  Mowat
Geoff  Rathbone      John Vale-Taylor   Michael  Wyles

Any other member interested in adding their name should contact Phil Berry or Geoff Rathbone.

The event is free apart from the planetarium although members need to make their own travel arrangements.

Details of the visit were given in the March issue of the Newsletter and will be included again in the June Newsletter.


The "Help List" on a clipboard is available at each meeting and is for members to use when asking for help or information.  This is a useful way of introducing problems being experienced and queries by members.


There is to be a SAGAS Summer Meeting on Saturday 19th June 2008 and it is being hosted by the Basingstoke Astronomical Society.

It is a long way and the cost is 10 but there are talks and trade stands.

The event is open from 1300 until 1900 and members of the Wadhurst Astronomical Society are welcome although there is limited space.

More information is available through the link to be found on the SAGAS website at: www.sagas.org.uk/



Mercury is at its most favourable this month as far as evening observing is concerned; however early risers in October will see the best morning apparition. By mid May Mercury will set two hours after the sun at around 22.50 BST and can be found low down near the west-north-west horizon at magnitude +0.4. If you intend to sweep for it with binoculars, make sure that the sun has set before you do so.

Venus is too close to the sun for observation this month.

Mars at magnitude +1.3 by the middle of the month is still fading in brightness as the distance between us increases. The red planet crosses the border between Gemini (the twins) and Cancer (the crab) as it moves steadily eastwards.

Jupiter at magnitude -2.4 in the constellation of Sagittarius (the archer) will be low in the sky for the entire year. It is a morning object rising around 01.00 BST in the middle of the month but only manages a maximum 17 above the horizon when due south.

Saturn lies in the constellation of Leo, close to Regulus (a Leonis) the bright star at the bottom of the asterism that looks like the mirror image of a question mark. On May 3rd it is stationary before beginning its eastward motion once more. At magnitude +0.6 it is still a prominent evening object but by the middle of May it will set just before 03.00 BST.

Lunar Occultations

Reasonably bright occultations are a bit thin on the ground this month. As usual I've only included events for stars down to around magnitude 7.5 that occur before midnight BST. The disappearance on May 13th is of 58 Leonis and should be an easy event with even a small telescope. All times are in BST. DD = a Disappearance at the Dark limb.
May Time Star Magnitude Phase PA degrees
10th 2054 SAO 80164 7.3 DD 153
11th 2155 SAO  98517 6.6 DD 77
12th 2342 SAO 118171 7.4 DD 136
13th 2355 SAO 118610 4.8 DD 72

Lunar Occultation of Mars

On May 10th there will be a daylight occultation of Mars around lunchtime. The planet will disappear on the dark limb of the moon at 13.14 hrs BST and reappear on the bright limb at 14.02 hrs BST. Although scientifically this type of event is not remarkable it is certainly interesting for the amateur to watch.
New First Quarter Full Last Quarter
5th 12th 20th 28th


Below are details of the most favourable passes of the ISS this month that occur before midnight as seen from Wadhurst. The information given is for when it is at maximum altitude, so it is best to look a few minutes before this time. Full details of visibility can be found at: - www.heavens-above.com 

Times are all BST.
Date in May Magnitude Time Altitude Azimuth
22nd -1.9 2229 36 SSE
23rd -2.5 2248 67 SSE
24th -2.5 2308 85 N
25th -2.4 2153 61 SSE
25th -2.3 2327 76 N
26th -2.5 2212 88 N
26th -2.5 2347 87 NNW
27th -2.3 2232 76 N
28th -2.4 2116 88 S
28th -2.4 2251 84 N
29th -2.2 2136 76 N
29th -2.5 2311 68 SSW
30th -2.4 2155 81 N
30th -1.8 2330 37 SSW
31st -2.5 2214 74 SSW

Brian Mills


Stellar Compass for Space Explorers

by Patrick L. Barry

In space, there's no up or down, north or south, east or west. So how can robotic spacecraft know which way they're facing when they fire their thrusters, or when they try to beam scientific data back to Earth?

Without the familiar compass points of Earth's magnetic poles, spacecraft use stars and gyros to know their orientation. Thanks to a recently completed test flight, future spacecraft will be able to do so using only an ultra-low-power camera and three silicon wafers as small as your pinky fingernail.

"The wafers are actually very tiny gyros," explains Artur Chmielewski, project manager at JPL for Space Technology 6 (ST6), a part of NASA's New Millennium Program.

Traditional gyros use spinning wheels to detect changes in pitch, yaw, and roll-the three axes of rotation. For ST6's Inertial Stellar Compass, the three gyros instead consist of silicon wafers that resemble microchips. Rotating the wafers distorts microscopic structures on the surfaces of these wafers in a way that generates electric signals. The compass uses these signals-along with images of star positions taken by the camera-to measure rotation.

Because the Inertial Stellar Compass (ISC) is based on this new, radically different technology, NASA needed to flight-test it before using it in important missions. That test flight reached completion in December 2007 after about a year in orbit aboard the Air Force's TacSat-2 satellite.

"It just performed beautifully," Chmielewski says. "The data checked out really well." The engineers had hoped that ISC would measure the spacecraft's rotation with an accuracy of 0.1 degrees. In the flight tests, ISC surpassed this goal, measuring rotation to within about 0.05 degrees.

That success paves the way for using ISC to reduce the cost of future science missions. When launching probes into space, weight equals money. "If you're paying a million dollars per kilogram to send your spacecraft to Mars, you care a lot about weight," Chmielewski says. At less than 3 kilograms, ISC weighs about one-fifth as much as traditional stellar compasses. It also uses about one-tenth as much power, so a spacecraft would be able to use smaller, lighter solar panels.

Engineers at Draper Laboratory, the Cambridge, Massachusetts, company that built the ISC, are already at work on a next-generation design that will improve the compass's accuracy ten-fold, Chmielewski says. So ISC and its successors could soon help costs-and spacecraft-stay on target.

Find out more about the ISC at nmp.nasa.gov/st6.  Kids can do a fun project and get an introduction to navigating by the stars at:


This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.



Chairman   John Vale-Taylor 

Phil Berry  01892 783544

Treasurer  Mike Wyles  01892 542863

Publicity & Website  Michael Harte  01892 783292

Newsletter Editor  Geoff Rathbone  01959 524727

Any material for inclusion in the June 2008 Newsletter should be with the Editor by May 28th  2008