Lunar X and V Times for 2021

Lunar X and V Times for 2021

The Lunar X and V are transient Clair Obscur effects which are visible on the lunar surface once a month for about four hours. The “X” is caused by light illuminating the rims of craters Blanchinus, La Caille and Purback. The “V” is caused by light illuminating crater Ukert along with several smaller craters.  The X is at its most striking when it is visible on the shadow side of the terminator (as shown above) but they will remain visible against the lunar surface for a few hours even after the terminator has moved over them if you know where to look (see photo below). The X is located about a quarter of the way up from the bottom, just inside the shadow side of the terminator. The V is approximately half way up just inside the illuminated side of the terminator, and it really shines bright against the darker background of Mare Vaporum. If you know where to look, you will be able to spot them with binoculars (it will help if they are mounted) but they are best viewed through a telescope.

The X and V are visible some time close to the First Quarter phase, however, due to libration, the exact time that they are visible is different from month to month. The lunar phase illumination during which they can be seen during 2021 varies between 41% and 55% illuminated.

To figure out what time the X and V become visible each month, I used the NASA Scientific Visualisation Studio Moon Phase and Libration tool for 2021 (https://svs.gsfc.nasa.gov/4874 ) I did this by scrolling through hour by hour near to First Quarter until I could see the X become visible; the V tends to become visible a little earlier than the X. This is an approximate start time, and they should be visible for a few hours after this.  There is no fixed end-time listed because as mentioned above, these features remain visible even after the terminator moves across them, but if you assume they are visible for around four hours from the start time, you will see them at their best.

Although the X and V occur every month, the time may be before the Moon has risen or after it has set from your location, so you will not see them every month. Sometimes they are visible on a daytime Moon, when they are much more difficult to observe and photograph. The times given are in 24 hour clock and are in UT/GMT (and BST where appropriate) so you will need to correct for time zones and daylight time savings changes if you are not in the UK. I have also included the approximate moonrise and moonset times in the table. These times were for London, UK so your exact rise and set time will vary depending on where you are in the UK. You can check sunrise and set times for your location here:
https://www.timeanddate.com/moon/uk/london   

Date	Start	Moonrise	Moonset	Visible UK?	% illuminated
20th Jan	18:00 UT	11:12 UT	00:51 UT	Y	49%
19th Feb	08:00 UT	10:08 UT	02:01 UT	Y rising day	46%
20th Mar	22:00 UT	09:00 UT	02:00 UT	Y	43.5%
19th  Apr	11:00 UT 12:00 BST	09:10 UT 10:10 BST	03:27 UT 04:27 BST	Y day	42%
18th  May	23:00 UT 00:00 BST	10:12 UT 11:12 BST	02:32 UT 03:32 BST	Y setting	41.5%
17th  Jun	11:00 UT 12:00 BST	11:50 UT 12:50 BST	01:37 UT 02:37 BST	Y rising day	42.5%
16th  Jul	23:00 UT 00:00 BST	12:16 UT 13:16 BST	00:16 UT 01:16 BST	Y setting	45%
15th  Aug	10:00 UT 11:00 BST	14:13 UT 15:13 BST	23:21 UT 00:21 BST	N	47.5%
13th  Sept	22:00 UT 23:00 BST	14:50 UT 15:50 BST	22:29 UT 23:29 BST	Y setting	51%
13th  Oct	10:00 UT 11:00 BST	15:53 UT 16:53 BST	23:27 UT 00:27 BST	N	53%
11th  Nov	23:00 UT	14:01 UT	22:53 UT	N	55%
11th  Dec	12:00 UT	13:01 UT	00:30 UT	Y rising day	55%

Technically, there are nine months of 2021 when the X and V are visible. However, as stated above, Clair Obscur effects are difficult to observe on a daytime Moon, but they are also harder to observe when the Moon is very low because of atmospheric disturbances.  In February, June and December they will be visible on a rising daytime Moon and this will be a challenge for most observers. In July and September, they will be visible on a night-time Moon but only for a short time before the Moon sets.  By far the best months to see the X and V will be January and March.

It’s really great fun to observe how the Lunar X and V regions evolve over time, so if you do make the effort to see them when they first appear, make sure you check that region again periodically to see how things have changed.  The sketches below show how different the Lunar X region can look once the terminator has passed over it. If you enjoy observing the X and V, there many other Clair Obscur effects that are well worth seeking out. There is comprehensive list of them on Wikipedia here.

I really hope you found this post helpful. Please feel free to share it with anybody who may find it useful.

Clear skies!

Mary McIntyre FRAS