Castle Point Astronomy Club
Castle Point Astronomy Club Diary
June 2017 by Dave Stratton
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7th June 2017

Andrew introduced Nick James for his talk:

What Have We learned about Comet 67P?

Nick began by reviewing some typical comets including Hyakutake and Hale Bopp.

The project with 67P was to learn what was happening in the nucleus.

The Giotto Probe in 1986 had a go with Halley’s Comet and was the first spacecraft to make close up observations of a comet.

Nick passed around a model of 67P made on a 3D printer. He said its very strange shape, which is reminiscent of a bathtub duck, is because it was formed from two bodies that coalesced. He showed us a clip of how this may have occurred.

He explained that comets are basically dusty snowballs which orbit the Sun. They are made of ices, such as water, carbon dioxide, ammonia and methane, mixed with dust.

He described the Rosetta Orbiter which is a similar size to an A320 plane. The craft is 2.8m X 2.1m X 2.0m with solar panels each 14m long.

The Philae Lander which sadly crash landed and was ‘lost’. It gained some charge and came back to life a little but not enough to do work or enable it to be found. Nick said that Philae was about the size of a suitcase weighing 100kg.

Nick explained that Rosetta was using fuel to maintain its orbit which it could not do for long so a decision was made to land it on the comet.

Nick explained the programme since the study began.

Rosetta used star field tracking and the dust around 67P caused problems at perihelion so it had to back off.

He explained that the period of 67P is 6.4 years and it loses material each time so it has a finite life.

Rosetta was launched toward 67P to land at a specific spot. But remember that the comet is rotating and the journey time was several hours. As Rosetta approached many pics were taken at high definition.

Nick said the escape velocity was 70cm/sec so you could jump off it – literally.

As Rosetta descended images were taken the last being from 20m. One image contained a shot of Philae - so it was found!

He said that a great deal of science had been gathered. The comet nucleus is not spherical and this alters the affects of sunlight which upsets the ‘seasons’. Sunlight varies by a factor of two.

The comet is very dry 1% with up to 4% in a few bright spots.

It is thought that it formed closer to the sun and migrated outwards.

Its surface temperature reduced by 20k due to sublimation.

The few active areas are ‘turned-on’ by sunlight.

67P is largely made up of dry dust. However the dust is much denser than the nucleus indicating that the nucleus must be porous.

Nick showed us a graph of the range of size of pebbles/boulders from mm to 50m.

Outbursts (changes in brightness) were caused by cliff collapses that expose fresh material which react and cause the outburst.

We saw some delightful images that showed some cliffs before and after collapse.

Nick explained that at every perihelion the comet loses 1m of surface on average but locally it can be 15m and the sublimation causes the rotation to slow by 20 minutes. One rotation takes 12.5 hours.

We saw several images showing the changes to certain locations over time.

The comet contains molecular oxygen which implies that it has stayed cold.

He explained with a chart how the early solar system worked.

It is thought that the two lobes collided at 20m/sec. The structure has very little rigidity it is just held together by very weak gravity.

The comet is likely to last for 6,500 years.

Nick mentioned two current comets V2Johnson and Comet 2017 K2 Panstarrs which is currently 16AUs away, beyond Saturn. It will potentially be bright in 2023.

We saw a pic of V2 Johnson taken by Nick from Las Palma and one by our very own Peter Carson taken locally.


14th June 2017

Andrew said that next week, which will be the longest day, we would have Telescope Zoo – this will not be an observing session as that would be silly. Instead we will just look at the scopes.

Peter said this was a good time to look for Noctiluscent Clouds in the Northern sky. These are ice clouds that are very high and visible long after the Sun has set.

Gerald will have the solar scope out again as he did this evening.

Mike said that later on Ed would be having a look at Jupiter.

Mike said he was giving the talk this evening on:


He began by posing the question ‘What are Orbits?’

He asked us to consider the thought experiment where you have a canon on a mountain top.

When the canon is fired the ball will eventually hit the ground – but what if the canon was very powerful and could fire the ball so hard that it would travel right around the world. He explained that although this was a fanciful idea it helped us to understand that when an object is in orbit it is actually falling at the same rate as the world´s surface is curving away.

Arthur C Clark predicted Gio Stationery orbits. So that when the speed of orbit is just right the object will remain above the same spot on the Earth.

The faster the object is travelling increases the height of the orbit and vice versa the slower the object the lower it is.

Mike explained Kepler´s three laws:

The path of the planets about the Sun is elliptical in shape, with the centre of the sun being located at one focus. (The Law of Ellipses)

An imaginary line drawn from the centre of the Sun to the centre of the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas)

The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the Sun. (The Law of Harmonies)

He told us that although the orbit is an ellipse the area swept out in a given time will always be the same.

We saw a chart showing the proportionality of the planets´ orbits.

We learnt about Conic sections – a slice at an angle through a cone is an ellipse – however a circle is an ellipse with an elliptability of zero.

An ellipse has an elliptability of 0 – 1 say 0.66. A parabolic orbit would have a velocity greater than the Escape Velocity.

We had a bit of a shock at this point as Mike split us into several groups and said we were going to actually draw an orbit of an existing object in space.

He said we would need to understand the Orbital Elements.

To give some understanding of what this involved I have reproduced here courtesy of the internet the laws.

I expect you find it all clear now!!!

Anyway with a lot of support from the expert (Mike) we constructed orbits of Comet Encke, Asteroid Apophis and Comet Holmes.

Basically we drew ellipses with some cotton and two pins, calculated the inclined angle and cut out orbits and inserted them into the plane of the Earth´s orbit.

At end of the day they looked really good.


21st June 2017

This week we are having our annual event:

Telescope Zoo

This is the longest day of the year so the worst possible date to use a telescope so instead of looking through them we look at them.

We had a pretty good selection of instruments along the largest being the club´s 16inch Dob which is looked after by Ron and the smallest being a 60mm refractor courtesy of Kevin.

In addition we had a range of scopes and a pair of large binoculars.

Tony had brought his son Thomas along for the first time who loved it and wanted to become a member there and then. Ted said as it was so late in our season with only four evenings left he said Tony could be considered to have converted to a family membership for free for the remaining sessions.

A good deal of honest to goodness nattering went until later we were able to see Jupiter in the South we observed it through Kevin´s refractor and Ed´s 10inch Dob.

Thomas was very interested to see his first planet and asked how far away it was. We had a little lesson with his dad being the Sun me the Earth and Thomas was Jupiter to understand that Jupiter was six times further away from the Sun than we were.

That’s what it´s all about.

28th June 2017

Andrew said that next week we have a Stikfest and that the village fayre was on the 15th July. Help is needed as Ted is away.

Ed said that Sandra, Bernard Hobbs´s daughter has advised that Margery, his wife, would like to sell Bernard´s stuff – details to come.

Mike said he was giving the presentation this evening and the subject was:


All were demonstrated live via the internet or from a downloaded copy on a tablet. Many of them are free and if not free not much dosh required.

A wealth of info - amazing.

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