The 2016 transit of Mercury

On the 9th of May a rare event will be visible from the UK. Mercury will be crossing the face of the Sun and the whole event will be visible from the UK, assuming the skies are clear. The transit starts at 12:12BST and ends at 19:41BST. Although the whole event last 7.5 hours, the most interesting parts are the 3 minutes when Mercury starts the transit and the 3 minutes at the end when it leaves the disk of the Sun. For those with H-alpha scopes you might get a few extra minutes if the tiny planet should happen to cross a prominence on its way in or out.

Mercury transits occur around May or November, about 13 or 14 times a century. So although more common than a Venus transit, they still unusual. And given the weather in the UK, this one is the best one you’re going to get for quite sometime. The last one visible from the UK was back in May 2003. In November 2019 and November 2032 we will get to see partial transits (weather permitting, which is unlikely given the typical winter weather for the UK). The next full one won’t be until November 2039 and it only lasts 3 hours.

On May 9th first contact, when the disk of Mercury first encroaches on the Sun’s disk occurs around 12:12BST, as shown in the diagram below:

Mercury transit 1st Contact

Second contact, when the trailing edge of Mercury’s disk comes onto the Sun’s disk, occurs about 3 minutes later at 12:15BST. Third contact is when the leading edge of the planets disk starts to leave the Sun’s disk and it occurs at 19:39BST as shown below:

Mercury transit 3rd contact

Fourth contact is when the disk of Mercury leaves the Sun’s disk completely at 19:41BST.

It is worth starting to observe these events a few minutes before these times as they can vary be a minute or two depending on where you are in the UK. You will need some sort of optical aid to see this as Mercury is too small to be seen with the naked eye. Do NOT look as the Sun using a telescope or binoculars, without using appropriate filters. The safest method is to project an image of the Sun through a telescope of binocular lens onto some card. Abingdon Astronomical Society will be holding a public observing event, see here for details. Do come and join us if you get the chance.

Clear skies,
Ian

Notice of AGM

The societies AGM takes place on the 9th May 2016 at 8:30pm (BST). Please note the later starting time. Due to the organisers finishing off at our public observing event of the Mercury transit on the same day, the AGM will start at 8:30pm. If you get there any earlier you’ll have a half hour wait.

We have a place on the committee if you would like to become more involved in the running of the society and we have had one nomination for chairperson of the society. If you’re interested in either then please contact Ian Smith on ian@ismith.org.uk or on 07557 373401.

After the AGM, society member Gwyneth Hueter will be telling us about her recent eclipse trip to Indonesia.

Clear skies,
Ian

February’s talk: Infra-red astronomy

Dr Eric Dunford is a space scientist from RAL. We do get great support from RAL and even though the topics sometimes overlap there is always much knowledge of interest to be had. This time the subject was ‘Infra-Red Astronomy from Space’.

Dr Dunford started working on astronomical satellites in the late 60s and he gave us a run through of the various IR spaceborne observatories that have had a European and personal interest to him.There are problems with observing in IR, in that the atmosphere’s water vapour stops most wavelengths of IR, and also that spaceborne observatories will in themselves give off heat radiation, some of which will be IR. This makes it essential for these satellites to have sunshields, and detectors have to be supercooled so that their own radiation doesn’t mess up the observations.

The first, and very successful one was the Infra-Red Astronomical Satellite (IRAS) in 1983. This was in the days before CCDs, and it had lots of different detectors of three different wavelengths. The mirror was 33 cm in diameter and the instruments were kept at a temperature of 2.5 kelvin, thanks to the gradual evaporation of a helium jacket. The data was downloaded every 12 hours and the helium evaporation ended up keeping IRAS going for 319 days, leaving us with observations of 400,000 objects. It is still out there, orbiting at 900km from Earth. It also discovered a comet which was also discovered visually, Comet IRAS-Araki-Alcock. Vega was used as a calibration object but it turned out to be a flop because it had a protoplanetary system around it so was not a stellar point source.

The IR Space Observatory (ISO) survived for about 2.5 years and had a 60 cm mirror and detectors giving 1000x more sensitivity than IRAS. Dr Dunford then explained that it was very difficult to do any testing on these kinds of gadgets prior to launch as you had to test them cold, i.e. 2.5 kelvin on Earth, and that made all the materials brittle.

Herschel was the next IR telescope with European interests and was launched in May 2009.It is still the largest mirror we have put into space, 3.5m. It was launched with Planck (a far IR and submillimetre telescope he did not touch on) on an Ariane 5 rocket. It moves around in the L2 point (in a line from the Sun through the Earth/Moon). It had three detectors (SPIRE) observing in wavelengths longer than what IRAS used, going into far infra-red to submillimetre wavelengths. It was able to detect water on Mars.

The PACS detector looked at shorter wavelengths (70 to 160 microns) and were used more to look at stars and galaxies. The resolution of M31 and the Rosette Nebula was stunning. The ground based James Clerk Maxwell Telescope in Hawaii was able to observe in these wavelengths but its field of view was minute in comparison. Herschel ran out of fuel in April 2013 but the data is still being analysed.

The James Webb Space Telescope, about which we heard much in our January speaker meeting, will also be observing in the near infra-red. Dr Dunford was pleased to tell us that the last telescope segment was installed in the week before his talk.

Gwyneth Hueter.

January’s talk: Exploring and Observing Asterisms

Thank you again to Wiltshire AS for providing another speaker for us: Jonathan Gale, ‘Exploring and Observing Asterisms’
He is a member of SPOE = Salisbury Plain Observing Group.

Asterism is originally from French, and is defined as a prominent group of stars within a constellation, not necessarily a chance alignment of stars.

Some asterisms are a nice easy base for starhopping, such as the Summer Triangle or the Square of Pegasus. The latter is a good test for light pollution. You might see six stars within the Square from here; from Devon you might get 17.
Some asterisms are sectional i.e. they belong to part of a constellation, such as the Keystone of Hercules, Orion’s belt and the Sickle of Leo.

Some of the asterisms are small, such as the Coathanger (Collinder 399, and remarked on by Arab Astronomer Al-Sufi in 964) and Kemble’s Cascade (NGC 1501/2). Nice to hear a little about Fr Lucien Kemble, a Canadian night sky observer (1922-1991).
Mr Gale took us through various asterisms to look for over the seasons, including some with colours, such as the upside down sailboat around 22 in Leo Minor and the Dolphin’s diamonds coming off the nose of Delphinus.

David Ratledge (who I knew when I was a member of Bolton AS in the 1980s) has a hefty list of asterisms on his own www.deep-sky.co.uk. Go and have some fun trying to look for them. And why did no one mention the Teapot?

Gwyneth Hueter.