The Meridian laser marks the route of the Greenwich Meridian by night in a northerly direction from the Observatory at Greenwich.
People often ask us: from how far away it can be seen? Not surprisingly, the answer to this is highly dependent on the weather and the atmospheric conditions at the time. But under good conditions it's a lot further than you might imagine. The furthest at which it has been reliably sighted with the naked eye is at a distance of 36.7 miles from Periwinkle Hill a couple of miles to the south of Royston.
In order to see the beam when more than a few miles away from the Observatory, you need to be standing directly beneath it (and definitely no more than a few hundred yards or so to either side), and looking south, back along the beam towards Greenwich. There are a number of elevated sites in Cambridgeshire to the north of Royston with good horizons to the south. My good friend and former colleague Robin Catchpole visited one such a site at Lolworth (a small village just to the south of the busy A14) at the end of last year. Although not visible to the naked eye, Robin was able to see the beam with his binoculars and photograph it with his compact digital camera. During the first weekend in January, when much of southern England was enjoying clear days and frosty nights, I joined Robin on the Saturday evening to go laser hunting slightly further to the north in the village of Bluntisham at a distance of 60.5 miles. Once again, the laser was visible but once again, only though binoculars ... and here's the picture that Robin took to prove it.
At this sort of distance from the Observatory, the visibility of the beam is highly dependent on the state of the atmosphere at all points along its route and unfortunately for us, within just a few minutes of our arrival the beam disappeared from view. Under exceptional conditions, I would hazard, that it would be possible to see it from as far north as Bluntisham with the naked eye.
If you want to go laser hunting yourself, you will need either a suitable Ordnance Survey (OS) map and or a GPS. Because of the way the maps are constructed, the Meridian doesn't run vertically up the page. Instead it curves slightly from right to left. Although most OS maps do not specifically mark the Meridian, it is possible to determine more or less where it runs by making use of the graticules (the small blue crosses) that are generally marked every 5' of latitude and longitude on the Landranger maps - the ones with the purple covers. What you will need to do, is look along the top edge of the map where the longitude scale is marked until you find 0o and then look down the page for the corresponding graticules. What you may also find useful (especially if you need to use binoculars), is a magnetic compass to help orientate yourself in approximately the right direction. The Periwinkle Hill site can be located fairly easily even if you don't have an OS map or GPS as the beam passes almost directly over the transmitting tower that is located there.