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

Venus's Close Encounter with The Pleiades

Venus’s Close Encounter with The Pleiades

Venus and Messier 45 the Pleiades by Mary McIntyre, 2nd April 2020

The path of Venus during the first half of April. Map created by Mark McIntyre using Star-Charter

During the first week of April, observers will be treated to an awesome spectacle in the western sky after sunset.  The stars within Messier 45, the Pleiades, will be joined by the extremely bright planet Venus. This open cluster which is also known as the Seven Sisters, is a firm favourite amongst astronomers because the gas and dust around the young, hot and blue stars are very striking in long exposure photographs.  

Venus moving through the Pleiades - 1st - 5th April.
Animation created by Mary McIntyre using Stellarium and PIPP

Venus is currently blazing around mag -4.5 and on 2nd, 3rd and 4th of April it passes very close to the Pleiades. The animation above shows the position of Venus from 1st – 5th April. On 3rd April it is at its closest and will look like an extra “star” within the cluster. The picture below shows the closest approach on 3rd April.  

The position of Venus and the Pleiades at 21:00 BST 3rd April 2020

Screenshot from Stellarium

It’s not unusual for Venus to be visible in the same part of the sky as the Pleiades, but it’s only once every 8 years that it passes this close to the cluster, and it always occurs in early April. This is due to the orbital resonance between Venus and the Earth. This ratio is 13:8, which means that Venus orbits the Sun 13 times in the time it takes the Earth to orbit 8 times. The next close conjunction will occur on 4th April 2028.

The conjunction can be seen with the naked eye, but it will look even better if you observe it with a pair of binoculars or a small telescope and it will make a fantastic photo opportunity!  Venus and the Pleiades will become visible in the West after the Sun sets. The best time to view them will be around 21:00 – 21:30 BST but they will remain visible until they set at around 23:30 BST.

Mary McIntyre FRAS
www.marymcintyreastronomy.co.uk

Messier 45 the Pleiades by Mary and Mark McIntyre, October 2016

Lunar X and V Times for 2020

Lunar X & V Times 2020

The Lunar X and V are transient lunar features which are visible on the lunar surface for about 4 hours, once a month. They are caused by sunlight illuminating the edge of craters. 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.  They are at their most striking when they are visible on the shadow side of the terminator, but they will remain visible against the lunar surface even after the terminator has moved because they are brighter than the surrounding area.

Lunar X & V - 11th May 2019 by Mary McIntyre

The X and V are usually visible a few hours before First Quarter phase, however, due to libration, the exact time of the X and V being visible is different from month to month. I initially used the Lunar Terminator Visualisation Tool to get the approximate times of the X and V, then as I did in previous years, I used the NASA Scientific Visualisation Studio Moon Phase and Libration tool for 2020 (https://svs.gsfc.nasa.gov/4768) to check the start times. I did this by scrolling through hour by hour on the date of the X and V to ensure I had the most accurate times for the X and V becoming visible.  The times I recorded are listed below.  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.

The Lunar X and V on the illuminated side of the terminator, 5th May 2017
By Mary McIntyre

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. They may also be visible during daylight, and they are more difficult to observe and photograph on a daytime Moon.  Please also note that the times given are in 24 clock and are in UT (the same as GMT) so you will need to correct for time zones and daylight time savings changes. I have included the approximate moonrise and moonset times from Oxfordshire, UK and a note about when the X and V are visible from the UK.  

Date	Start Time of X & V	Moonrise	Moonset	Visible From UK?
2nd January	20:00 UT	11:50	23:50	Y
1st February	10:00 UT	10:50	00:55	Y (Day)
1st / 2nd March	23:30 UT	09:35	01:00	N
31st March	13:00 UT / 14:00 BST	10:10	03:10	Y (Day)
30th April	01:45 UT / 02:45 BST	11:00	03:20	N
29th May	13:45 UT / 14:45 BST	11:20	02:20	Y (Day)
28th June	01:30 UT / 02:30 BST	13:10	01:30	N
27th July	13:00 UT / 14:00 BST	13:40	00:15	Y (Day)
26th August	00:30 UT / 01:30 BST	15:35	23:45	N
24th September	13:00 UT / 14:00 BST	15:45	23:20	Y (Day)
24th October	01:30 UT / 02:30 BST	15:51	00:30	N
22nd November	14:30 UT	13:45	23:40	Y
22nd December	04:30 UT	12:45	01:00	N

There are seven occasions during 2020 when the X and V are visible from the UK, but unfortunately only two of those occasions coincide with a night time Moon, the first being on 2nd January from 20:00 UT. The second occasion is on 22nd November when they first become visible during daylight, but by 17:00 UT it should be dark enough to make it easier to spot them.  However, if you have good binoculars or a telescope you will still able to observe the X and V on a daytime Moon, so please do try to find them on those dates too.

The Lunar X & V up close 11th May 2019 - by Mary McIntyre

Mercury Transit - 11th November 2019

Transit of Mercury 2019

On Monday 11th November 2019, we have the opportunity to observe a rare event; Mercury transiting the disc of the Sun. The last time we saw this from the UK was on 9th May 2016, but the next transit will not take place until 2032.

A transit occurs when a planet whose orbit is closer to the Sun than Earth, passes between the Earth and Sun, and the planet can be seen as a small black disc crossing the Sun’s surface. As viewed from Earth, the only planets that transit the Sun are Mercury and Venus. Because the orbits of these planets are slightly tilted, they do not cross the Sun’s disc every time they orbit. Approximately 13 times per century Mercury transits occur, but Venus transits are rarer. The last Venus transit took place in 2012, but the next one isn’t until 2117. The reason Mercury transits are more common is because Mercury is closer to the Sun and orbits more rapidly than Venus. Mercury only takes 88 Earth days to orbit the Sun whereas Venus takes 225 days to complete one orbit.  That said, a Mercury transit is still a pretty rare event.

How to observe the transit:
On 11th November 2019, Mercury starts to move across the disc of the Sun around 12:30 pm (GMT) and continues throughout the afternoon, leaving the disc after sunset.

Graphic showing Mercury’s position at 11:45am and 3:45pm on 11th November
Created by Mary McIntyre using Stellarium.
Note: we are unlikely to see this many sunspots on Monday!

SAFETY NOTICE: NEVER EVER ATTEMPT TO LOOK AT THE SUN EITHER DIRECTLY, OR THROUGH BINOCULARS OR A TELESCOPE WITHOUT THE CORRECT EQUIPMENT. YOU WILL GO BLIND! DO NOT UNDER ANY CIRCUMSTANCES TRY TO USE ECLIPSE GLASSES WITH BINOCULARS OR A TELESCOPE; THE MAGNIFICATION OF THE SUN WILL BURN STRAIGHT THROUGH THE GLASSES AND CAUSE PERMANENT DAMAGE TO YOUR EYES. IF YOU ARE USING A TELESCOPE, MAKE SURE YOU KEEP THE CAPS ON THE FINDERSCOPE. EVEN THAT LOW MAGNIFICATION IS ENOUGH TO BURN YOUR EYES AND CAUSE PERMANENT DAMAGE.

The Sun is so bright that even a glance at it through a telescope or binoculars can cause blindness. So you need a full-aperture filter of suitable material such as Baader AstroSolar on the front of your telescope or binoculars. Baader AstroSolar filter material is available from many suppliers in A4 sheets for about £20.

If your telescope has a lens cap that has a smaller removable cap, you can make a filter that sits inside the larger cap, making sure the smaller aperture cap is completely covered with solar film. It’s important to check that no light is getting through around the edges and that there is no damage anywhere on the filter.

Alternatively, you can buy glass solar filters, like the one in the image below made my Thousand Oaks. Whichever filter type you use, make sure it is well-secured with no risk of the filter blowing or falling off. 

A selection of solar filters

If you have a suitable filter in place, you can observe the Sun directly through the eye piece. Because Mercury is so small (it will be a tiny dot!), you will need a magnification of about around 50x or more. Because Mercury is so small, you will not be able to observe the transit using eclipse glasses or small binoculars with solar filters. To give you an idea of the apparent size of Mercury against the solar disc, see the image below taken during the 2016 transit.

Using a DSLR and Zoom Lens:
If you have a DSLR camera and a 300mm (or greater) zoom lens, you can make a solar filter to sit on the front of the camera to photograph the transit. If you do this, make sure you use the live view screen to achieve focus; never look through the view finder! You will find it easier if you cover the back of the camera and your head with a black blanket to keep out residual light while you try to get the camera focused. Below is the set up I used for the total solar eclipse in the USA back in 2017.

Home-made DSLR Solar Filter

Telescope Projection Method:
If you do not have time to make or buy a solar filter, you can use a projection method. But do not attempt this method unless you know what you’re doing. Do not attempt this method if you have a large aperture reflecting telescope because the parts may get too hot. Also make sure that all of your eye pieces and eye piece holders are not made of plastic, because they will melt in the heat of the Sun.

First of all, you need a piece of white card, or white paper glued onto a piece or card, or a piece of white board. This will be your projection board. If you can, try to find a way of mounting it onto a tripod or broom handle so it is free standing. It will help you greatly later on if it can stand up by itself.

Begin by lining up your telescope with the Sun.  Place your projection board behind the eye piece and use the finderscope to help you get things lined up properly. If you are using a refractor, your projection board will be behind the telescope, but if you are using a reflector (as shown in the image below) it will be at the side. When everything is in place, you will see a large bright disc on your projection board. Adjust the focus on your telescope until the disc is nice and sharp. You should easily be able to see any sunspots which are currently visible. Once you have aligned the telescope, immediately put the caps back onto the finderscope because even that small amount of magnification can cause permanent eye damage if you accidentally look through it. During the transit, you will see Mercury as a tiny black dot slowly passing across the projected Sun’s disc. If your telescope does not track, make sure you constantly make adjustments to keep the Sun in shot, otherwise in the internal components of your telescope may be damaged. If you want to take a photograph of the projection on the white board, it is safe to do so, but do not try to take a photograph by holding your camera up to the eye piece - it will burn your camera sensor! Never leave a solar projecting telescope unattended in case somebody who doesn’t understand the danger tries to look through the eyepiece.

Using a 4.5” reflector to project the Sun

At the time of writing (4th November) the weather forecast is not looking good for my location in North Oxfordshire. But let’s hope that we get at least some brief cloud gaps and get a chance to witness this rare event. Wherever you observe this event, good luck and happy observing!

Mary McIntyre FRAS
Oxfordshire, UK

Visit my website: www.marymcintyreastronomy.co.uk
Facebook: Mary McIntyre Astronomy
Twitter: @spicey_spiney
Instagram: spiceyspiney
Flickr: spicey_spiney
You Tube: Mary McIntyre FRAS