Projecting onto an oblique curved surface

I went to see the brilliant Katherine Priddy play in a tent. The blue spotlights were circular so the beam must have started circular. It hit the tent canvas in the middle top of the photo, spreading out into an almost parabolic shape. A very clear visual mapping from a circle to a parabola. 

Vapour trail

There is a whisp of vapour trail in the middle of the picture. I watched the plane pass. It seemed to go through the high cirrus cloud. Why did the contrail persist there and not elsewhere? Is it that the humidity is high there so it can't evaporate? Is that why there is a cloud there in the first place?

Tare

 I have been aware of the TARE button on electronic balances for several decades but thanks to Mrs B for pointing out the definition in an old dictionary from over a century ago, as being the mass with the container removed. In recent years, it has often been labelled as the "zero" button. Place the container on the balance and press it to make the reading set to zero. It's interesting that it had a usage similar to algorithm. And its other usage: I thought tares were generic weeds but it seems to be specifically Vetch.

Slight aurora

I was out at 9.30pm on an Amber alert and was able to photograph a faint aurora. I was not able to see anything directly with my eye.

Below is another 9.30pm image

At 1.30am after a Red alert, even the camera was struggling to pick up the aurora. The effect is very variable.