Copernicus

It's a short hop from Eratosthenes to Copernicus and this gave me a chance to get a contrasting picture to the full-moon shot I took a few months ago.

Here are the pictures side by side:




As I mentioned in my July post, the bottom picture is misaligned - the crater in the bottom right is the one at the top edge of the other picture. Eratosthenes just drops off the bottom of the full-moon shot.

The misalignment doesn't matter much since the object of interest is Copernicus itself. The shadows really show up the 1km depth of the crater. Actually I think a little less shadow would have given a better result as more of the crater floor would have been visible. Copernicus is a young crater and has lots of features on the crater floor that would have shown up nicely.

Eratosthenes

My next stop along the half-moon terminator was Eratosthenes. Named after the father of geography, inventor of the system of latitude and longitude, and all round Greek genius, Eratosthenes is located on the south edge of Mare Imbrium at the western end of the Montes Appenninus. It's a relatively deep crater that catches the eye due to it's prominent location and proximity to the crater Copernicus.

Similarly to my observations of Plato described in my previous post, I was dodging light cloud for much of the session. Here's the best shot:


Eratosthenes is the crater towards the bottom-left. The depth of the crater and terraced inner rim are clearly seen. There are also multiple central peaks visible and little evidence of lava flooding which suggests the crater may be younger than flooded craters such as Plato. In fact Eratosthenes is believed to be around 3.2 billion years old and its formation marks the start of the Eratosthenian Period which is partly characterised by reduced volcanic activity.

Looking closely at the bottom-left quarter of the picture it is possible to see the ejecta from the neighbouring crater Copernicus, which show up as lines of lighter coloured material. The three craters at the top of the picture, moving left to right, are Timocharis, Archimedes and Autolycus.

Plato

After much too long a break I've finally managed to get some new Moon pictures. Despite some intermittent wispy cloud and a sudden onset of fog I got a fairly good run at a near half moon.

Starting from the north, this post concerns Plato (the favourite crater of Space-1999 fans). As suggested by having a famous name, Plato is one of the most distinctive lunar craters. Those with good eyesight can spot it with the naked eye as a dark patch near to centre-top of the lunar disk. Plato is about 100km in diameter and has a dark, lava filled floor. It is located at the western end of the mountain range Montes Alpes and between Mare Imbrium (the Sea of Showers) and Mare Frigoris (the Sea of Cold). Plato is nearly 3.8 billion years old and slightly younger than Mare Imbrium to the south.

Here's the best picture of the bunch (which isn't bad considering there was never more than a couple of minutes between clouds):


The angle of the sunlight shows the irregular rim and jagged peaks casting shadows across the crater floor.

For completeness, the crater on the terminator towards the top of the picture is Fontenelle. The four craters forming an arc in the top-right are Anaxagorus, Epigenes, Timaeus, Archytas and the large, flatter crater in the top-right corner is Goldschmidt. The mountain to the south of Plato is the 2.4km high Mons Pico.

Sunspots

There's still not much action on the sunspot front but at least there is usually something to look at pretty much every day. A couple of weeks ago I pulled the camera out again for a few snaps.


I thought I'd also have a go at zooming in on one of the sunspots using the webcam (in this case the one towards the bottom of the disk in the image above). I had the usual problem of achieving focus throughout the video capture but some detail shows up.

Altair

Last month I tried a few different ideas with my Star Analyser diffraction grating. One of the bigger successes was taking some black and white images as well as the usual colour shots. Since it's usually best to experiment on something that's easy to find and that this was still summertime, I settled on Altair.

Altair is one of our near neighbours, about twice the mass of the Sun and ten times as luminous. Similarly to the other two points of the summer triangle, Altair is of spectral class A so has some fairly strong Balmer lines that should show up nicely using the Star Analyser. Here's what I ended up with:


Both sets of spectra have their merits but the black and white trio show much more detail. The colour pictures show a clear H-Beta absorption line (in the light blue) but the H-Alpha and H-Gamma lines (in red and blue, respectively) have to be searched for. The black and white shots not only clearly show these three absorption lines, but also show the H-Delta line.

It is also much easier to locate the peak luminosity when looking at the black and white images. Looking at the colour images the peak luminosity could lie anywhere in the light blue, green or yellow. The black and white images quite clearly show luminosity peaking in yellow.

I Got The Sun In The Morning...

...And all of the rest of the day since I now have an Orion Solar Filter. Simply fit it over the front of the telescope and it cuts out 99.999% of incoming light. This makes the Sun a safe observing target and shows up sunspots, prominences and granulation.

Sunspots are areas on the surface of the Sun where the magnetic field has become tangled up making one patch cooler than the surrounding area. The temperature of the sunspot will be around 4,000 degrees compared to around 6,000 degrees for the surrounding area. The cooler area shows up as dark spot on the surface. Sunspots can be very big - the biggest can be up to around 50,000 miles across.

It has been well publisized that the Sun has been rather quiet over the last few years. The Sun goes through a fairly regular cycle every 11 years or so where it changes from having very few sunspots to lots of sunspots and back again. The current low period has been going on for longer than usual but there are signs that activity is beginning to pick up again (last year 71% of days showed no sunspots compared to 16% so far this year).

This increase in activity left me optimistic that once the recent spell of rain had abated I would be spotting spots straight away. Alas no. The clouds cleared at the start of a mini-streak of 5 spot-free days. This morning was a different story. After consulting the latest space weather, I knew that I'd have at least one sunspot to look at, and here it is:



Sunspot 1101 is clearly visible in the bottom-right quadrant. There was quite a lot of whispy cloud around (as can be seen in the picture) so I settled for a few afocal snaps rather than getting out the webcam. It will be interesting to see what resolution can be achieved on a clearer day and what happens to sunspot 1101 in the future.

Gamma Cassiopeiae

With autumn approaching I felt it was about time to turn my spectroscopic attention to Cassiopeia. The first stellar spectrum I ever observed was that of Gamma Cassiopeiae and 15 years later I couldn't wait to see how it would look through my Star Analyser.

Back in 1866, Gamma Cassiopeiae was the first star ever observed with emission lines in its spectrum. This made it the prototype Gamma Cassiopeiae Variable star, the first known Be star (a type B star with emission lines), and one of the most popular targets for spectroscopy ever since.

One of the most common features of a stellar spectrum is to see hydrogen absorption lines. These are caused by hydrogen atoms in the star absorbing light at a wavelength corresponding to the energy required to excite an electron between different energy levels. For example, the spectrum of Sirius that I posted last month shows a clear H-Beta absorption line caused by exciting electrons between the second and fourth energy levels.

Here are three spectra I took of Gamma Cassiopeiae and instead of absorption lines we can see a clear H-Alpha emission line (in the red part of the spectrum).


Clearly some other process must be taking place. Some of the great minds of the early 20th century found that these emission lines must be coming from material around the star rather than the star itself. This material is a disc produced by a combination of very rapid rotation, magnetic fields and stellar pulsing. The cooling of hydrogen atoms in this disk then produce the emission lines seen in the stellar spectrum.

When I observed this star at university (with considerably better equipment!) I could produce a much more detailed spectrum. It was possible to measure the rotation speed of the disc by calculating the Doppler broadening of the emission line. It was also possible to see a narrower absorption line in the middle of the broad emission line caused by the disk absorbing light from the central star.