Relative speed

Here is the Queen Elizabeth passing the identical Queen Victoria out in the Atlantic. We'd been tipped off that it was about to happen. Things happen slowly at sea but this happened twice as fast as expected. We were steaming north at 20 knots. She was steaming south at 20 knots. That means that the distance between us was shrinking at 40 knots. That was our relative speed. To us, we could have been still with the Elizabeth approaching at 40 knots. To them, the Elizabeth was still and we were approaching at 40 knots. That was all fine until Einstein got hold of it and started thinking about what would happen when you get to the speed of light .... but that's another story.

Full Moon on Christmas Day

Here it is, the first full Moon on Christmas Day since 1977, as seen somewhere out on the North Atlantic. I wondered why it had taken so long and what factors affect how often any given date gets a full Moon. It turns out that there is more than one way to measure a complete orbit of the Moon around the Earth. One is to measure a complete 360 degree rotation, but you have to do this against the background of the stars and it turns out to take 27.3 days. It is called the sidereal month. My next task will be to figure out how to observe this. More logical is the month from New Moon to New Moon ie measuring the time it takes between occasions on which the Moon and the Sun line up. Here it rotates more than 360 degrees round the Earth because the Earth itself has moved on round the Sun. It takes about 29.5 days but varies during the year because the Earth's orbit is not round. This is the synodic month. 12 lots of 29.5 days comes out at 354 days, so each full Moon will slip by 11 days in a regular year but 12 days in a leap year. I started trying it out on a calendar but gave up. Anyway, next full Moon on Christmas Day is something like 2034 so it will be a while.

Volcanic insulation on Gran Canaria

The southern end of Gran Canaria is very arid. They have put in a lot of tourist apartments by the sea. The flat roofs seem to be covered in the local volcanic rock. I assume that this is insulation to maintain a cooler temperature inside, which is the opposite of what we need up north in the winter. The rock is porous due to gas bubbles in the lava flows and thus it traps air. This makes it a poorer conductor of thermal energy thus causing it to take longer for the thermal energy to get through into the house.

Nine o'clock Oxford time

I had a room opposite Tom Tower in my second year. The bell rings 101 times at five past nine in the evening which was a nuisance at times! I understood that it was to summon the students back in for the night but I could never understand why five past nine. It turns out that it is a result of G.M.T. When the railways made a common time for the UK vital to keep trains running on time, the idea of using the Sun highest in the sky in that town as noon went out the window. Time was now fixed by noon at Greenwich. Noon at Greenwich is five minutes ahead of Oxford, so by ringing at five past nine G.M.T. the bell was still being rung at the true 9pm local time.

Brilliant programme about Einstein

To celebrate 100 years of General Relativity, the BBC have made a great documentary. Here's the link http://www.bbc.co.uk/programmes/b06s75vs It will be valid for a couple of weeks.

Max Min Thermometer

 I got a wonderful present from Mrs B at the weekend - a Max Min Thermometer. This is also known as Six's Thermometer after its inventor. It is a U-shaped thermometer which records the same temperature on both sides. If you look carefully you'll see that the number scales go in opposite directions on each side. Each side also has a small steel marker in the tube. These record the maximum and minimum temperatures in a given time period. They stay where they are and you have to use a magnet to pull them back to the end of the coloured liquid to allow the process to start again.

 It works because there is a clear liquid in the tube that records the minimum temperature (left hand side above and right hand side below). This expands and contracts with temperature pushing the blue temperature recording liquid round. Above the blue liquid on the other side is a vacuum - nothing to stop the blue liquid moving round the tube.

Earthshine - lighting up the Dark Side of the Moon?

Look at this picture of a crescent Moon, taken around dawn. The bright crescent is all we can see of the half of the Moon that is facing the Sun, given the angle that we are viewing from. You'd think that the side facing the blackness of space would be dark. But you can see it because sunlight reflects from the Earth back up onto the Moon, lighting it dimly. It's like the full Moon shining on Earth - that was bright enough for us to walk in the hills last year. Now I am aware that the phrase Dark Side of the Moon doesn't mean the side out of the light - that's another story - but it sounded nice in the title!