Tuesday, 11 May 2021

Noctilucent Clouds and How to See Them

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!


NLC panorama with Comet NEOWISE


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.  


Bright NLC

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.

Cirrus vs NLC ice crystal size












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.

NLC 18th July 2015













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.

NLC Panorama 3rd July 2018



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.


NLC Panorama 21st June 2020

NLC Panorama 3rd July 2017




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 22nd June 2020, the morning that I saw NLCs reaching all the way to the zenith from Oxfordshire. (Source: spaceweather.com)

NLC Daisy 22nd June 2020












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!


NLC Ripples 6th July 2016













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

Sunday, 3 January 2021

Lunar X and V Times for 2021

Lunar X and V Times for 2021

Lunar X and V photos

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.

Lunar X and V photo

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.

Lunar X and V November 2020

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

Pastel Sketches of the Lunar X and V


Tuesday, 31 March 2020

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


Friday, 20 December 2019

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