Castle Point Astronomy Club
Castle Point Astronomy Club Diary
March 2017 by Dave Stratton
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1st March 2017

Mike gave the sad news about Ed´s wife Ann with details of the arrangements.

Andrew said the BBC wanted to interview a bunch of astronomers about an invention that they would like to see. Let him know if you are interested.

Peter said that MikeB had asked if there are any more candidates for our next camp.

Also he said there was a coronal hole on the Sun so perhaps we will get some aurora.

Plus we should look out for Jupiter as it is rising at 21.30.

Mike introduced Gord Falconer for his talk:

The Why (and some Ys) of Eclipses

Gord put up a slide immediately to explain his strange title - History, Observability, Geometry, Frequency and Predictability (each ends in a Y).

He began by explaining Bessel´s Method of eclipse prediction according to Roberdeau Buchanan.

The basic concept is that Besselian elements describe the movement of the shadow cast by the occulting body - for solar eclipses this is the shadow of the Moon - on a specifically chosen plane, called the fundamental plane. This is the geocentric, normal plane of the shadow axis. In other words, it is the plane through the Earth´s center that is perpendicular to the line through the centers of the occulting and the occulted bodies. One advantage, among others, of choosing this plane is that the outline of the shadow on it is always a circle, and there is no perspective distortion.

We got a bit of history next from the 3rd Millennium BC. Apparently if the king got an eclipse that he was not expecting the court philosopher stood a fair chance of getting paying the ultimate price. Gord said that Thales of Miletus predicted an eclipse for 28th May 585 BCE. However there is some doubt that he had the ability at the time.

Gord said that eclipses ran in cycles. It is important to recognize that it is not how often they occur but how often they can be seen.

Lunar eclipses can be seen from anywhere by everyone. Solar eclipses happen just as often but the path of totality is very narrow. A partial eclipse can happen without being noticed. Even with a total eclipse the changes are minimal until totality is almost reached.

In Southend dduring the 20th century there were 42 eclipses at some level, of these, none were total and only four managed 90%. There were 166 lunar eclipses of which 60 were total.

Gord mentioned the Saros Cycle: The periodicity and recurrence of eclipses is governed by the Saros cycle, a period of 6,585.3 days (18 years 11 days 8 hours). It was known to the Chaldeans as a period when lunar eclipses seemed to repeat themselves, but the cycle is applicable to solar eclipses as well.

Gord explained with the aid of some diagrams that the Moon´s orbit is not co-planer. It is tilted by 5.1450 which means it crosses the plane of the ecliptic (where the Sun is) twice each month. We can only get an eclipse when the Moon is crossing the ecliptic. Gord said the Sun moves by almost 10 per day. There is a period of 34 days centred about the node when the paths of Sun and Moon are close enough for an eclipse to occur.

The line of nodes precesses westward by 190 per year whilst the Sun is moving 10 per day eastwards. This means the sun can reach the next node after 173.3 days (less than 180).

There are always two eclipse seasons in a year, there can be two eclipses per season and often four counting both solar and lunar.

The actual delay from season to season is 173.3 - 34 equals 139 days.

Successive eclipse will always be very different - see paragraph on Saros Cycle above.

The period of time it takes for the Moon to complete an orbit from one node to the same node is known as the ‘Draconitic Month’. The Draconitic month is slightly shorter than the Synodic month. The main reason for this is that during the time that the Moon has completed an orbit around the Earth, the Earth (and Moon) have completed about 1/13th of their orbit around the Sun.

Gord mentioned that the Saros Cycle means that after the 6,585.3 days the cycles will repeat. He also mentioned there is the Exeligmos cycle which is three Saros Cycles but suggested that was best left to another talk!

Gord showed us four slides depicting the tracks of solar eclipses from 1999, 2017, 2035 and 2053. They are all across the US. (In fact the event in 2017 is the much acclaimed August 21st one that several club members are going to attend). They have very similar tracks and are 18 years apart and demonstrating the Saros Cycle.

At any time there are about 40 Saros Cycles in progress.

Gord finished with some pretty pics starting with a stunning Hybrid Eclipse, this is when an eclipse is annular at one location and total at another. We saw two images of the same eclipse taken 3000km apart. Stunning images - Gord said we get typically six hybrids per century. Also another of an annular as it was setting and finally one more annular taken in Argentina on 26th February 2017 (a few days ago).


8th March 2017

Andrew reminded us about the ‘Outreach’ thing he is interested in.

Ted said he had organised some flowers for Ann.

MikeB said the club camp in two weeks was now sorted.

Andrew introduced Jim for his talk:

My 10 inch Travel Scope

Jim said that on a trip to Oz some time ago he realised the advantages of a scope designed for travel.

So he made one - a Dob. He had brought it along for us to see. It is very nicely presented being very squat with eight struts to make it rigid and the boxes are made of wood painted red.

He said that a Dob is actually a Newtonian Reflector, with the aid of diagrams and the scope he explained how they work in principle.

He used UK Astro buy and sell to get the mirror. He said it is similar to EBay but it specialises in astro stuff so it is easier to trust it. He got a GSO mirror (made in Taiwan) for £100 which he tested and is pleased with.

With the aid of a diagram he explained how he tested the mirror and said that a rolled edge was a problem but a defect in the middle did not matter as that part is not used as the secondary mirror covers it. He used a beam splitter so that the laser illuminated the whole of the mirror.

Jim showed an image of a mirror through a Rounchigrame grid (I think) showing the pattern of lines.

He described how a Dob works and said the sliding surfaces used bumpy Formica sheets and Teflon/PTFE pads which give a good slide without friction.

He explained that the modern Dobs were much squatter than the earlier models. The scope was open sided to reduce weight but they need a shroud to cut out glare if there are nearby lights.

He explained the design of the rocker box and we saw a pic of his set-up in his garden to establish the actual focal length and establish the length of his truss poles. He ended up with eight to improve the rigidity.

He uses a Red Dot finder.

His initial design used bungee cords to prevent the scope from falling over, but these got replaced by a rather neat spring system.

His structure is made of wood which works fine but will cause a problem if it goes to Australia as they do not allow wood to be imported.

He showed sketches of his new project which is a Schmitt Cassegraine and then a 24inch Dob. (He is struggling with a 20inch at present!).

Andrew introduced MikeB for his talk:

Recognizing The Night Skies

Mike by saying that due to the Earth´s spinning the sky appears to rotate each 24 hours and it also changes with the seasons. He took us quickly through how this works.

He also explained what sidereal time was. Briefly sidereal time is a time scale that is based on Earth´s rate of rotation measured relative to the fixed stars rather than the Sun.

Polaris is almost at the celestial pole and is a major help in orientating the sky.

He loaded Stellarium and set it to show the sky rotating and the constellations moving.

He explained that we can find stuff by using a system similar to latitude and longitude called RA and Dec. He said that RA was in fact time and Dec was the angle from the Equator.

We often use the term ‘object’ in astronomy - this is anything in the sky.

He mentioned star catalogues of which there are several: The Messier has 110 objects, Caldwell has 109, NGC has 8000 and IC has 4000 in addition to NGC.

There are 88 constellations and they do not look like their names.

Mike said it was important to learn the sky. He found a simple Planisphere very useful and there are many sky programmes available - including Stellarium - which is free. He used to just pick a constellation and study it - what was in it? He found the Phillips Night Sky Atlas to be very good and the Cambridge Star Atlas.

He gave a demo on his tablet which was connecting to our projector via Bluetooth so he could hold the tablet up to the ceiling and on the screen was what the tablet was ‘seeing’.

He described in principle how imaging was done with the aid of an iOptron Sky Tracker. It is a device that can carry a camera and follow the sky. An AstroTrac will do similar.

He mentioned that red dot finders were a great aid to pointing the scope correctly.

He said that getting the focus correct was hard so he suggested using the Moon or Venus to set focus was good. He also said The Bahtinov mask is a device used to focus small astronomical telescopes accurately. It was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005.

He gave us some nice images of stuff he has done including star trails, ISS, meteor showers, Milky Way, Earthshine, North American and Pelican Nebulae, Orion´s belt plus M42, Barnard´s Loop, Flame and Horsehead Nebulae, Heart and Soul Nebulae, M41, M31 and M13.

Wonderful - If that was a ‘Frantic-Fill-In’ we should have them more often.

15th March 2017

Barrie said he was raising money for a charity and had a box of stuff for all to consider.

Ed was having a clear out and had a miscellany of mainly computer stuff including a colour printer and loads of recordable CDs, books etc. and it was all free.

Andrew said we were having a: Stikfest

Jim was up first with IC1396A the Elephant´s Trunk nebula is a concentration of interstellar gas and dust. We saw a stunning image of the nebula which is 3 - 4 moons across. The two bright stars, close together, are causing a shock wave. This was the result of 20hours of imaging as it´s very faint.

Then he had a nice crescent image of Venus. As Venus is an inferior planet, inside our orbit, it always shows a phase. It is currently getting closer, in line of sight, to the Sun and we will shortly be treated to the phenomena that it will be both an evening ‘star’ and a morning one at the same time. Jim finished with a single channel image of 1396.

Peter had an image of Comet 21P - Giacobini-Zinner it is a periodic comet with a circular orbit at Jupiter-like distance. It has variable brightness. Peter said he had imaged it over 3 months - on 19th August it was very faint but by 22nd it was much brighter. It is thought that this is caused by an outburst of material from within which results in the brightness increasing.

Peter explained that he uses a system that compares the brightness of known stars nearby to that of the comet and judges the subject object from them.

Jack had some stuff on his phone that a bunch of clever folk were needed to get the phone to communicate with the laptop. Which despite their doubts they managed to do!

He began with images of the 10.4 m (34 ft) Great Canary Telescope, dedicated 24 July 2009. Both general views and from within and explained what we were looking at. He also had a video that involved turning the phone he had connected to our projector to show it, being held on its side to get the picture the right way up!

He also had pics of the Meridian line at Stratford. He said if you wanted your picture taken there it was a much better bet than Greenwich where you had to pay for the privilege. However it should be noted that the actual meridian is not there anymore it is actually 102 meters to the East! But it´s still history.

Jack also had a pic of a set of Brandon eyepieces that he considered buying at AstroFest. We also saw a pic of a ‘dead’ lamppost (lying on its side) - I failed to record why! And he showed a pic of a Celestron 130 scope complete that was at the Rayleigh community tip! He finished with some images of Einstein´s papers collection.

Interesting and different.

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