Post Solar eclipse 2015

Fri 20 March 2015

I managed to see quite a bit of the eclipse this morning, despite the clouds. My children and I observed it from the balcony at the front of our house using a pin-hole camera made out of a cardboard washing powder box. We called it the HGT.

The HGT - Hubble Ground Telescope. Several pin-holes were made on the front and images projected onto the white paper stuck on the back. The altitude of the Sun at the time of this photograph was about 25° which, apparently, equates to an Ikea plate+Sky remote+Wiimote.

From the start of the eclipse at 08:30 to nearly 10:00 the sky was mostly overcast and cloud obscured the Sun (sometimes known as a Glasgow eclipse). There were however some brief moments in which the cloud thinned and the dimmed crescent Sun could be glimpsed without dazzling the eyes.

By 09:30 - 4 minutes before the local maxium of 95% was reached - the light outside took on an eerily dark feel, and outdoor lights with sensors and most car headlights came on. I noticed also that the hallway inside the house, lit only by daylight coming in from other rooms, seemed oddly dark. This peculiar light lasted until about 09:40.

Normal sunlight will provide about 1340 W of power across a square metre surface facing it. That's more than a bar of an electric fire or about twenty-two 60 W light bulbs. So, given that at the maximum of this partial eclipse the sunlight was reduced by 95%, dropping the power to 67 W per square metre, it may seem puzzling that the darkness wasn't more noticeable. The explanation is that the change in light is gradual enough that our eyes have time to adapt to the new light levels. It's only when you get close to 100% that your vision fails to compensate and it will start to seem dark. Also, because cloud cover can change much faster than the solar eclipse progresses, its effect is often more marked.

From 10:00 am onwards the clouds parted sufficiently to allow the pin-hole camera to work. It soon became obvious that the hole I'd made was too large, so I used some finer pins to make a few smaller holes. I had to take quite a few photographs with my phone to get one that was steady enough to see the eclipse clearly - the best one (and yes, it's not I know it's not great!) is this one:

Pin-hole images. Projected images of the Sun at 10:20:52 photographed with the camera on my Nexus 4 phone from a slightly oblique angle.

The first hole was too big and gave the over-exposed and blurred image at bottom right. The second hole's image is top left and is also blurred because it was too big. The smallest holes produced the middle two images which are sharper and fainter. They compare well with what Stellarium shows for that time:

Stellarium shows the eclipse at 10:20 on 20 March 2015.

I do have access to more advanced equipment for solar observing, including telescopes with solar filters. So why did I choose to go with the primitive pin-hole approach? Because it's simple, there's little to go wrong and that makes for an experience that's more fun and interesting. In fact, the best moment, although it was fleeting, was when we saw the crescent Sun through the cloud with nothing other than our eyes. That was special.

The next solar eclipse visible in the UK won't be until 2017, and it'll be much less impressive. But if you're eager for more eclipse action, then the next lunar eclipse, in which the Earth's shadow covers the full Moon, will be on 28 Sept 2015.