From time to time, we have to check and adjust the alignment of the Meridian laser. Rather conveniently, back in 1824, the sixth Astronomer Royal John Pond had an Obelisk erected at Pole Hill some 11 miles from the Observatory on the then Greenwich Meridian.

Chingford-Obelisk.jpg

Its purpose was to provide an additional alignment check for the Transit Telescope that was then in use. Having initially adjusted the telescope to the Meridian from observations of the circumpolar stars, Pond lowered it to the horizon to determine exactly where the obelisk should be built. By this means, he was able to position it with an accuracy of about ± four inches. The Meridian defined by this particular telescope is known as the Bradley Meridian and is the Meridian used (for historic reasons) by the Ordnance Survey (OS) on its maps. It predates the current Greenwich Meridian, which runs about six metres further to the east at Greenwich and is defined by the Airy Transit Circle (ATC) of 1850. An OS trig point was erected adjacent to the Pond Obelisk and very close to present Meridian in the 1930s, and this is where I will be heading later this month.

From there I will be able to talk with my colleagues back at Greenwich and tell them in which direction (east or west) they need to move the beam. The beam will also be adjusted vertically so that it just grazes or clears the tops of the trees, thereby ensuring maximum visibility for those places further to the north. So if early one winter's evening, you think that you can see the beam moving around a bit, your eyes probably won't be deceiving you ... it will simply be us doing one of the regular adjustments that are sometimes needed!

Meridian-Laser-at-ATC.jpg

For those who want a bit of info about the laser itself, it's a Millennia VS Diode-Pumped, cw Visible Laser. It has a wavelength of 532 nm and an output power > 5W. The laser unit is located beneath the Airy Transit Circle and 'fired' along the Meridian from above via a fibre optic as can be seen in the picture above.