Popular Clair Obscur Effects: Times for 2023

Popular Clair Obscur Effects: Times for 2023

Lunar X & V, The Face in Albategnius and The Eyes of Clavius

by Mary McIntyre 

Please note that the tables showing the timings below are photos because I have been unable to copy the text into a table in Blogger without the text getting scrambled. The downloadable PDF of this document is available and the tables there contain text that can be interpreted by E-reading software. You can download the PDF here. 

Every year I produce a table showing the times that the Lunar X and V are visible for the coming year.  I know these are the most popular of the Clair Obscur effects but they are not the only ones, so this year I am also including times for the Face in Albategnius and The Eyes of Clavius. Hopefully this will give you something new to try to tick off your lunar observing list. Detailed information about these effects is below, but for those of you who are only interested in the data tables, I’ve put those first.  The moonrise/moonset times are for Oxford, UK, and will vary slightly across the UK. The times that the effects are visible are listed in UT so if you’re outside of the UK you can convert this to your own local time then and check the moonrise and set times for your area.

The Lunar X and V

The Face in Albategnius

Approximately 2 hours after the Lunar X is at its best, when the terminator has moved across to cover the area around it, take a close look at the shadows on the right hand side of the crater Albategnius (located almost halfway between the X and V). At just the right time the shadow looks like the side profile of a face.  This shows up more clearly on stacked photos which have better resolution and sharper features. The above image was taken with a William Optics 70mm refractor with Celestron 3x Barlow. The camera was an ASI120MC.

The Eyes of Clavius

Additional information if you’re new to these effects:

The times given in the data tables 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 if you are not in the UK. I have also included the approximate moonrise and moonset times in the tables. These times were taken from the Time and Date website and relate to my location in Oxford, UK. Your exact rise and set times 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  

To ascertain the times these effect appear each month, I used the NASA Scientific Visualisation Studio Moon Phase and Libration tool for 2023 (https://svs.gsfc.nasa.gov/5048)

Lunar X and V

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). 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 brightly against the darker background of Mare Vaporum. Once you know where to look, you will be able to spot them with large binoculars (it will help if they are mounted) but they are best viewed through a telescope. They will show up on photos taken with a 300mm zoom lens or through a modest telescope.

They will remain visible against the lunar surface for a few hours even after the terminator has moved over them. (see photo below).

Left: The lunar X and V seen at their best.

Upper right: The X and V can still be seen for a short time when the Sun has risen over those craters, but it isn’t as striking as when the X is illuminated against the unlit surface

The X and V are visible 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 at the time they’re seen during 2023 varies between 43% and 54%. The time that they occur may not coincide with the Moon being above the horizon from your location, so you will not see them every month. 

From the UK, the Lunar X is visible seven times during 2023, but three of those apparitions are during daylight hours which are much harder to view, one is during twilight and one is when the Moon is close to setting. February and July are by far our best opportunities.

In previous years when I’ve observed the Lunar X using the start times from this tool, I have found it may take about 45 minutes from the start time before the X becomes clearly visible. The V tends to become visible a little earlier than the X. The start times are approximate, 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.

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. 

Pastel sketches by Mary McIntyre showing how the Lunar X stands out against better against the darker background before the lunar terminator has passed over it.

The Eyes of Clavius

Clavius is the second largest crater on the lunar nearside so it’s a brilliant crater to observe, even with modest equipment.  It is a roughly circular crater that has a diameter of 225km, but its location near the southern limb means it appears foreshortened top to bottom so it therefore looks like it’s an oval shape when viewed from Earth. Interestingly, there are very few truly oval shaped craters on the Moon; almost all oval craters only look that shape to us because they’re foreshortened by their position near to the limb.

Comparison of Clavius as seen from Earth and from orbit. Clavius appears oval from
Earth due to foreshortening

Clavius has several satellite craters along its floor. Clavius C and D, with diameters 21km and 28km respectively, have walls that are higher than the crater floor. As the Sun rises over Clavius, the tops of these crater walls catch the sunlight before the rest of the crater floor and this creates two white rings that resemble a pair of eyes looking out from the shadowy crater floor.

The Eyes of Clavius are visible during a Waxing Gibbous Moon, but as with other Clair Obscur effects, the exact phase angle varies each month due to libration. During 2023 they occur when the Moon is between 59% and 67% illuminated.

Left side: The Eyes of Clavius shine out from the shadowy crater floor as the Sun rises over Clavius.

Right side: Clavius fully illuminated with more satellite craters visible 

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 this or the printable PDF with anybody who may find it useful.

Clear skies!

Mary McIntyre FRAS

Astronomy Communicator

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Lunar X & V Times for 2022

Lunar X and V Observing Times for 2022 by Mary McIntyre

NB: The table below is an image because the plain text version keeps getting scrambled. I'm so sorry if you're using electronic reading software that can't translate this, but you can download a PDF of this blog which should be readable here. The PDF is also printable for anybody wanting a physical print out of this information. Please feel free to share this information with anybody who may find it helpful.

More Information if you’re new to the Lunar X and V

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). 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. Once 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.

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 lunar X can still be seen for a short time when the Sun has risen over those craters, but it isn’t as striking as when the X is the only illuminated thing in an otherwise unlit part of the surface.

The X and V are visible 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 2022 varies between 42% and 54%.  The Lunar X is visible twice during November so that brings the total number of apparitions during 2022 to thirteen.  

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 2022 which you can view here.

I did this by scrolling through hour by hour near to First Quarter until I could see the X become visible; in previous years when I’ve observed the Lunar X using the start times from this tool, I have found it may take about 45 minutes from the start time before the X becomes clearly visible. The V tends to become visible a little earlier than the X. The start times are approximate, 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 taken from the Lunescope app on my phone so they relate to my location in Oxfordshire, UK. Your exact rise and set times will vary depending on where you are in the UK. You can check sunrise and set times for your location on the Time and Date website here.

Although technically there are several months of 2022 when the X and V are visible from the UK, Clair Obscur effects are difficult to observe on a daytime Moon and they are also harder to observe when the Moon is very low because of atmospheric disturbances.  In May and December they will be visible on a rising daytime Moon and this will be a challenge for most observers. In August, October and the second apparition in November, they will become visible on the night-time moon but only for a short time before the Moon sets.  The best months to see the X and V will be February, April and June.

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. Once again, please feel free to share it with anybody who may find it useful.

Clear skies!

Mary McIntyre FRAS

Noctilucent Clouds and How to See Them

To see my You Tube video all about Noctilucent Clouds, click here

Noctilucent Clouds and How To See Them

By Mary McIntyre FRAS

www.marymcintyreastronomy.co.uk

June and July are difficult months for astronomers in the UK because have so few hours of darkness. Does this mean less sleep deprivation? Not a chance, because it’s Noctilucent Cloud season! Noctilucent clouds (NLCs) are one of the highlights of the summer calendar.  The 2018 NLC season was one of the best I’ve ever experienced, and the 2020 season was also very memorable because not only did I see a display where NLCs were visible up to the zenith, but also I had a display alongside Comet NEOWISE; I won’t forget either of those in a hurry!

What are NLCs?

Noctilucent clouds (NLCs) are also known as “night shining clouds” and if you’ve ever observed them you will know they certainly live up to their name.  Visible during deep twilight, they have a beautiful blueish-white ethereal glow.  They are very different from the tropospheric clouds that we see during the day.  Primarily, it’s due to their altitude.  Cumulus clouds are some of the lowest clouds we see and they reside around 2km above sea level.  Cirrus clouds, the thin clouds associated with ice halos, sundogs and other atmospheric optical effects, are the highest tropospheric clouds we see (excluding towering thunder clouds) and they reside at an attitude of around 6 km.  

Where are They?

To find NLCs, we need to go much higher. The boundary of the troposphere is 10km above sea level. Above that we have the stratosphere; a layer that extends to an altitude of 50km, and it’s unique to Earth. This is where UV radiation from the Sun breaks down oxygen into ozone to form the ozone layer. It’s also the home of a kind of polar stratospheric clouds called nacreous clouds, but they are rarely seen from the UK.  Above the stratosphere, we have the mesosphere and that extends to 85km; above that is the thermosphere. The boundary between the mesosphere and the thermosphere is where you’ll find NLCs. Aurora occurs just 15km above this, so NLCs really are on the boundary of space! NLCs are also seen in the tenuous atmosphere on Mars, where they reach a whopping altitude of 100km. This makes them the highest clouds recorded on any planet in the solar system. 

Formation and Visibility
NLCs are polar mesospheric clouds, so they cluster around the Polar Regions.  They are made of water ice, but here lies another huge difference between NLCs and tropospheric clouds; that is the size of the ice crystals. Cirrus cloud ice crystals that cause halos have a diameter of around 100 microns (0.1mm), so they’re pretty small. However, NLC ice crystals have a diameter of just 0.1 microns (0.0001mm) so they’re absolutely minute! This is why NLCs are not visible during the day. The lower levels of the atmosphere need to be in shadow and NLCs still illuminated from below by the Sun in order for them to be seen.

For NLC ice crystals to form, a source of water is needed. The mesosphere is very dry so NLCs may not form every day.  The mesosphere must also be below -123 degrees Celsius for ice crystal formation. The temperature of the mesosphere is colder when the troposphere is warmer, so NLCs can only form during the summer months. For us in the northern hemisphere, that tends to be from mid-May through to mid-August with the peak activity in June and July.  In the southern hemisphere, NLC season is mid-November to mid-February.

The ice crystals also need a nuclei to trigger crystal formation. NLCs had never been reported prior to 1885. Historically, excellent meteorological observations had been reported so it’s unlikely they would have been missed. Two years prior to the first reported display there had been a major eruption on Mount Krakatoa so it was thought volcanic ash particles were the catalyst. It’s now thought micrometeorite dust is responsible.

Link to Global Warming and Solar Activity
At one time NLC displays were rare, but they are now observed much more frequently. One theory is that climate change may be responsible. Human activity has pushed up the emissions of greenhouse gasses and that has lead to an increase in temperature within the troposphere. As stated above, an increase in temperature at that level causes a decrease in temperature up in the mesosphere. It has also been observed that NLC displays are brighter and more numerous when the Sun is at the minimum phase of the 11 year solar cycle. When the Sun is more active there is more UV radiation hitting the atmosphere. As well as breaking down oxygen into ozone, UV will also break down water molecules in the higher levels. Because the mesosphere is already quite dry and tenuous to begin with, UV radiation will remove all of the water and therefore no ice crystals can form. Solar scientists believe we are currently in a grand minimum so this may be another reason why we are seeing them more frequently.

How to see them
NLCs may be visible when the Sun is between 6 and 16 degrees below the horizon, because this is when the lower levels of the atmosphere are in shadow. NLCs are so high that they remain illuminated by the Sun so they appear to glow against the twilight sky.  Northern Hemisphere NLCs are visible from between around 45 and 60 degrees latitude. Any further north the Sun doesn’t get low enough below the horizon for the lower atmospheric levels to be in shadow.  Because they’re polar mesospheric clouds, they cluster around the poles. During the northern hemisphere NLC season, they cluster around the north polar regions. This means they are usually very low to the northern horizon from mid-latitudes, so only the largest accumulations of clouds will be visible from further south. 

NLCs become visible around 60 – 90 minutes after sunset and between 60 – 90 minutes before sunrise, but near to the solstice they may remain visible all night long. After sunset they tend to be visible towards the north west and before sunrise more towards the north east.  The amount of NLC visible in each display van vary enormously! Sometimes there is just a hint of white glow low in the north, other times they can stretch out over a hundred degrees across the northern horizon and reach all the way up to the zenith. The two panoramic images below show how different they can appear along the horizon.

They are usually silvery blue in colour (although they have been reported to appear as red and green) and can take on a huge variety of structures and patterns. Sometimes they are little more than a white glow, other times they have intricate structures that look like reflected ripples of water. They behave very differently from tropospheric clouds and they are absolutely stunning through binoculars. They are one of my favourite things to observe and they make a stunning timelapse subject.

Forecasting

There is still so much we don’t know about NLCs so they are very difficult to forecast. As they accumulate and rotate around the pole, they form a characteristic shape called the ~NLC Daisy” because it can resemble petals on a flower.  There is a satellite that observes and photographs this accumulation each day and if you visit the Space Weather website - spaceweather.com – you can find the most recent images on their front page. This gets updated daily and if you see any accumulation pointing towards the UK, then you’re in with a chance of seeing them. Remember, the accumulation may not look like it’s anywhere near us, but these clouds are so high that they will be visible from the UK. The photo below shows the daisy on 22^nd June 2020, the morning that I saw NLCs reaching all the way to the zenith from Oxfordshire. (Source: spaceweather.com)

There are several webcams across Europe and the UK that are pointing north (links are below). If NLC is visible from a location that is an hour ahead of us, then there is a chance that we may see them from the UK a bit later.  I find myself glued to these cameras during NLC season!

There is also a great community of NLC spotters on Twitter. If you follow @NLCalerts and @NLCnet and keep an eye on the #NLCnow hashtag, you will see observers reporting when displays are happening. If you see NLC yourself, don’t forget to report it to these outlets with your location to help spread the word of an active display. Also, don't forget to report your observations to the BAA Aurora/NLC section!

NLCs may be visible at very antisocial hours, but I think they are truly one of the most beautiful sights to behold and definitely worth staying up late or getting up early for!

To see more of my NLC photos, check out my NLC Flickr album: https://flic.kr/s/aHsmNjiAGt

To see my NLC timelapse video playlist on You Tube: https://youtube.com/playlist?list=PLE_LPip90NvY7PGahGXpQ5SxwJOiu_DmA

NLC webcam networks:
http://ukazy.astro.cz/nlc-monitor.php

https://www.iap-kborn.de/index.php?id=313&L=1

NB: I originally wrote this in-depth article for my blog page 7 years ago. Since then I’ve updated it every year and added new images. This year I’ve re-written the information completely and broken it down into subheadings so hopefully that makes it easier to read.

Other Sources:

The Cloud Book – How to Understand the Skies by Richard Hamblyn

http://en.wikipedia.org/wiki/Noctilucent_cloud

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