Saltcoats Solar Time

 It was 11.27am BST.

The lovely sundial in Saltcoats was at roughly 10.15GMT so actually about 15 minutes out.

I loved the phrase that my watch tells me "legal time".

Local time is set by when the sun is highest in the sky being 12 noon. You can tell by when the shadow on a vertical stick is shortest. Legal time has 12 noon fixed as being when the sun is highest in the sky in Greenwich. This fixing was necessary to make the time the same everywhere when railways appeared, needing fixed timetabling. It takes 24 hours for the Earth to rotate through 360 degrees so that's 15 degrees an hour. We were 4.5 degrees west so that's 18 minutes behind legal time, which is about right allowing for the width of the shadow.

The problem with my simple star projector

 

This lovely device projects the northern constellations onto the ceiling of my room. However I do have difficulty working out which constellation is which. I've realised what the problem is: All the stars are equal brightness. I'm used the differences in intensity between stars in a constellation and the simple hole method has me thinking I'm looking at a different sky!

Identifying the fossils

 Thanks to Tom for telling us what our fossils were and for showing us much better samples!

Below is Siphonodendron.

This is Lithostrotion junceum

They are both corals from the Lower Carboniferous period so maybe 350 million years ago.

The circle is a cross-section through the stem of a crinoid, which is sometimes called a sea lilly.

The correct ray diagram?

 

I should have thought to draw the Ray diagram first. Looks like both my reflections have had multiple reflections from both inner glass surfaces... 

Reflections in the double glazing

 

The first thing we noticed was that beyond the first pair of reflected beads there was a second fainter pair circled in yellow. The first pair must be the reflection from the inner pane and the second pair are reflected from the outer pane. 

Close examination of the nearest bead reflection shows a double row very close together. These will be reflections from the outside and inside surfaces of the single pane.

Looking from outside in, notice the reflection of the metal spacer between the panes.

Then using my finger as a marker, I detected a second reflection. This will be the reflection from the other pane of glass. It will be in this second reflection that I saw the thin-film interference bands last week.

Thin film interference cleaning windows

 

It hasn't photographed as well as I saw it but when I was spraying window cleaner onto the pane and wiping it off, the reflection of the double-glazing centre showed the sort of colour bands I've been thinking about. It seems that we might also be talking about multiple reflections from the inner surfaces of the glass as well so this is going to take some thinking about.

More ideas about thin film interference

 

I might have worked out why the colours seem more like secondary colours with thin film interference. Here's the ray diagram:

I am sure that the red ray that reflects from the lower surface ends up parallel to the red ray that reflects from the top surface. That means that they can interfere. Now the blue ray should have the same but having had a bigger refraction inside the thin film. This must mean that the red and blue rays can interfere with each other. Some wavelengths of colour will cancel so that not all will constructively superpose so we don't get a white reflection but if a few colours do constructively superpose, that would account for the strange colours.