If you have binoculars or a small telescope, then tonight (Wednesday 25th July) will be the perfect time to try and spot the mysterious Lunar X.
Look at the Moon tonight and you will see that the right-hand side is lit up while the left-hand side is still in shadow. The Moon is said to be in its first quarter stage. The second quarter is when it is full Moon, the third quarter is when the opposite sides are in light and darkness and finally the last quarter is a new Moon when the Moon is between the Earth and the Sun. The line between dark and light is called the Terminator (no, it is nothing to do with Arnold Schwarzenegger!) and it is along this line that the best features of the Moon can be seen with a telescope.
The mountains, craters, and valleys create long shadows on the Moon making them stand out more. Deep craters are still in darkness as the Sun has not risen above their edges and the tops of high mountains can be spotted in the darkened side of the Moon as the sunlight catches their peaks. Then if you observe the Moon over a period of a few hours, more and more of the crater or mountain will become illuminated as the Sun rises higher in the lunar sky.
And because there is no atmosphere on the Moon to scatter the light around like on Earth then the difference between the light and the shadowed areas is dramatic from pitch black to dazzling white along a very narrow line. And this is where the Lunar X comes in. As the Terminator creeps across the face of the Moon from right to left then different features slowly become visible as they catch the light. It was only in 1974 that someone happened to notice that at an exact period during the Moon’s first quarter a combination of light and shadows combined to form an X shape just slightly to the left of the Terminator edge on the darkened side of the Moon.
The X shape is created by three craters whose raised edges touch each other. Two of the craters are side by side whilst the third is above and directly between the other two. The raised edges of the craters are at least 800-1000 feet above the floor of the craters so they catch the sunlight while the surrounding area is still in darkness and so appear to form a distinctive X shape emerging from the darkness. But the effect only lasts for about four hours until the Sun has risen enough to illuminate the whole area. And tonight is a perfect opportunity for you to spot this amazing anomaly for yourself.
Now, it is small, so don’t go thinking that the X will suddenly appear as an apparition of biblical proportions on the face of the Moon. You will need binoculars or a telescope to see it and it will take some serious eyeball work to spot it as the whole area only covers about 50 square miles. So be patient and hopefully you should spot it. Begin by looking very carefully along the Terminator just below halfway down the face of the Moon. The upper part of the X should begin to appear at 22.36 and will gradually become more distinct as the evening progresses. I would guess that around midnight is when it will be at its most distinct.
Lee will also be showing other features along the Moon’s Terminator edge, including the Apollo 11 landing site in the Sea of Tranquillity, various mountains, valleys and the massive crater Tycho with its huge streaks that was formed when a giant asteroid crashed into the Moon many millions of years ago.
X marks the spot!
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Enjoy your Stargazing!