President’s Message – Nov 2021

Posted by as President's Message

Everybody should have a good astro-project on the go. My current one concerns the timing of lunar eclipses.

Solar eclipse geography and timing is known with remarkable precision. So much so that people, including many members of our RASC community, are willing to plan long, difficult, and expensive trips to watch them. The timing and location of the earth’s shadow, or umbra, across the Moon during a lunar eclipse is much more variable and poorly understood.

I was delighted to learn that as far back as 1687, Philippe de la Hire published that the Earth’s shadow was larger than could be accounted for by an airless Earth, leading to lunar eclipses that start a few minutes earlier and end a few minutes later than expected. This was important work, because observing the timing of eclipses was, in principle, one way to measure longitude – as long as the expected timing was well established.

Moon on Dallas Road, Oct 8, 2021, by Randy Enkin

The problem arises from the complex nature of the earth’s atmosphere that obscures, diffracts and refracts the sun’s light on its way to the Moon. I first became aware of the role of amateur lunar crater eclipse timing just before the eclipse last May (which was clouded out), and I am certainly keen to try again on the upcoming lunar eclipse, starting around 23:19 PST on Thursday, November 18. If there are clear skies, I’ll be out with my telescope, noting the time to the tenth of a minute that the earth’s umbra darkens (“immersion”) and then departs (“emersion”) from various lunar craters. Sky and Telescope has been compiling these observations since 1956. Herald and Sinnott (2014) have analysed the compilation, extended back to 1842, with an amazing 22 539 observations. Their main conclusion is that the Earth is surrounded by a “notional eclipse-forming layer” that is 87km thick. It is a really surprising result, since even noctilucent clouds don’t show up that high in the atmosphere.

Herald and Sinnott point out that amateur uninstrumented observations provide continuity with the early observations in their compilation and provide insight into the visual response of the human eye. To help with the observations, Thursday- Friday November 18-19, I have annotated a picture of the full moon with the crater timings predicted by Fred Espenak. I hope some of you will join me making these simple but useful observations.

Look Up,
Randy Enkin,

Comments are closed.