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 |
46% |
|
20th Mar |
22:00 UT |
09:00 UT |
02:00 UT |
Y |
43.5% |
|
19th Apr |
11:00 UT |
09:10 UT |
03:27 UT |
Y |
42% |
|
18th May |
23:00 UT |
10:12 UT |
02:32 UT |
Y |
41.5% |
|
17th Jun |
11:00 UT |
11:50 UT |
01:37 UT |
Y |
42.5% |
|
16th Jul |
23:00 UT |
12:16 UT |
00:16 UT |
Y |
45% |
|
15th Aug |
10:00 UT |
14:13 UT |
23:21 UT |
N |
47.5% |
|
13th
Sept |
22:00 UT |
14:50 UT |
22:29 UT |
Y |
51% |
|
13th
Oct |
10:00 UT |
15:53 UT |
23:27 UT |
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 |
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
