St Marys Harbour - 49 degrees and 55 minutes

The Scillies are the most southerly point in the UK and have made a thing out of the latitude. However, they are only 5 minutes off being 50 degrees north of the Equator. So how far is 5 minutes? The radius of the Earth is 6371km so circumference = 40030km. That's 360 degrees so 1 degree is 111.2km. 1 minute is 111.2/60 = 1.853km. 5 minutes is 9.266km which is comfortably enough to take in Tresco and St Martins meaning all of Scilly must be below 50 degrees.

What does 4000 square millimetres look like?

I'm always interested in units for measurement and was impressed to see a figure given in square millimetres. 1cm is 10mm so 1 square cm is 100 square mm. This means the area is 40 square cm. There are 1 million square mm in 1 square metre because it is 1000mm x 1000mm so the area is 0.004 square metres. It is circular so the diameter must be 40 = pi x diameter squared /4. Diameter = 7.1cm which is about right.

Data on my room for the night

We stayed in a shipping container. It had been beautifully done inside. I like the fact that there is data on the outside. The small panel has data called "Allowable stacking weight". Containers should be able to stacked up to at least 6 high. 192000/30480 = 6.3. This shows that the condition has been met even when the containers are fully loaded.

Safe working limit on the Scillonian

I'm always interested to see data in newtons. I had to look up S.W.L. It stands for "safe working limit". This is calculated by the maximum breaking load divided by some safety factor. 36kN is 36000N or just over 3.5 tonnes. I am still trying to find out what T.L. stands for in this context.

Bouyancy aid on the Meridian

I was interested to see that this life belt on Scilly is given a weight in Newtons. It is 100N but is suitable for people over 18kg. Such a person would have a weight of 180N so 100N must refer to the ring itself. It must have a mass of 10kg. I wonder whether the child having to be over 18kg is really about being big enough to grab the sides of the ring.

Batteries in parallel

I was interested to spot batteries in parallel in this LED garden light. It is designed to glow at night, so the solar panel cannot power the LED directly. During the day sunlight causes the solar panel to charge the lower battery. That then discharges at night through the LED. However, it might be that there is not enough charge on a dull day so the second non-rechargeable battery is there for when the rechargeable one has emptied itself. It doesn't add to the total output p.d. but is a substitute hence they are wired in parallel.

St Michael's Mount: bird on a wire

There were power lines across our view of St Michael's Mount and the birds came to perch. I was interested that although most chose to perch on the top wire that is the Earth wire and therefore carries no current, one perched on a lower wire. The theory is that since each foot is at the same potential, no current can flow through the bird because a potential difference is needed for current to flow. This is true but I seldom see birds perched on the lower wires - almost always on the Earth wire at the top.

Big Bang at Oxford University

I liked this mural on the wall of the new Physics Department building in Oxford. It seems to show the Universe from the Big Bang singularity on the right and expanding into the matter we have today.

Measuring the size of the Sun

It is very rare for me to get the opportunity to measure the Sun. The glare is usually too big. However at dawn the other week I was able to verify that the Sun subtends half a little finger at arm's length. That's half a degree or about 0.009 radians. Angle subtended in radians = diameter of Sun/distance to Sun. The Sun is 150 million km away so diameter of Sun comes out at 1.3 million km. That's big!

Wet ground conducts better

 I banged in 2 tent pegs on the campsite and measured the resistance of the parched earth between them. Off the scale means the resistance is higher than 2 million Ohms.

Then it rained. Notice how the resistance has come right down! (well, if you can call 506 000 Ohms low!)

Warming the stones at Brough Castle

 We visited Brough Castle on a warm day. The stones in the wall at the bottom of the wooden stair felt warm to the touch.I'm going to estimate warm as 30 degrees Celsius and say that it is therefore about 10 degrees above the air temperature.

The sandstone block is about 20cm long. Let's assume it is a cube of volume 8000 cubic centimetres. The density of sandstone is about 2.5 grams per cubic centimetre giving the block a mass of 20kg. Energy to heat it = mass x specific heat capacity x temperature rise = 20 x 920 x 10 = 184 000J. Assume the Sun was working at 1000 watts per square metre. The area of the block is 400 square cm so the block front gets 40W. It would take 184000/40 = 4600 seconds which is about 1.5 hours to heat the block. That's reasonable. It doesn't account for the conductivity of the block so perhaps the heated volume and thus mass is lower.