Moonwatch
“Northwest of Barrow sits Goldschmidt, with Anaxagoras to the west”
Crater Barrow lies close to the northern pole of the Moon. From Earth the consequence of this is the 93km-diameter crater appears extremely foreshortened as an ellipse and its appearance changes over time thanks to lunar libration. This is the term used to describe the fact that we don’t get to see exactly the same face of the Moon at all times. Variations in the Moon’s speed around its orbit combined with a 5° inclination of its orbit in relation to that of Earth, make it appear as if the Moon rocks and rolls on its axes. When libration tilts the northern limb of the Moon toward Earth, Barrow appears less foreshortened than when the limb is tilted away from Earth.
Foreshortened craters are harder to navigate than their more face-on counterparts because their ellipses have a tendency to merge into one muddled area. Barrow is surrounded by a number of craters that do a fairly good job of disguising it. When the phase allows, the easiest starting point is the dark, lava-filled crater Plato. With a diameter of 101km, Plato stands out clearly from the bright surrounding highlands. Move north from Plato across an area of highland material and you’ll end up in Mare Frigoris, the Sea of Cold. This is a highly elongated lunar sea, measuring 1,800km in length but just 200km in width.
On the northern shore of Mare Frigoris lies the small, well defined crater Timaeus, which at 33km is dwarfed by the massive walled plane of W Bond to the northeast. Barrow lies north and east of 160km W Bond, and is notable because of sharply defined 28km Barrow A interrupting its southwest rim. To the northwest of Barrow sits 121km Goldschmidt, with the prominent 51km ray-crater Anaxagoras immediately to the west. Ejecta rays from Anaxagoras can be seen to tiger-stripe the floor of Barrow when the Sun is high in Barrow’s sky.
Barrow’s rim is highest in the northwest where it connects with Goldschmidt and overall is very tortured, having been heavily eroded over time. The eastern portion of the rim has a break in it, which helps to form an interesting effect when the morning terminator is close by. At lunar co-longitude 2.5° the Sun isn’t high enough to illuminate Barrow’s floor, but can shine through the break in the rim. This creates a shaft of light that passes across the floor and is known as the Barrow Ray. When the ray is visible, a similar phenomenon can be seen in 54km Scoresby M to the north and in 56km Challis further north still. Libration has to be favourable to spot the Challis ray.
The Barrow ray should be visible around 17:00 UT on 24 March. The Sun will be up at this time but the effect should still be visible through a telescope. If you’re attempting to image the Barrow Ray, a mono high-frame-rate camera fitted with an infrared pass filter will give you the best contrast by darkening the blue daylight sky.