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Moon - 20250109

One of the rare occasions in the past 12 months where the moon has been visible late afternoon into early evening. Seeing was not especially good (Antoniadi III)  with a visible halo around the moon at sunset. It was very cold, quite humid, about -1C late afternoon falling to -5C by 2100UT. Condensation and icing of everything was a major problem and I eventually gave up when the primary mirror iced over.

Late afternoon, a IR pass filter 807nm  was used to exclude daylight wit the cost of greatly increased exposure times. The evening session this filter was replaced by a UV/IR block filter to assist with image sharpness. I am starting to thing that a V (green) band filter may be a better option, something to try next time.

The moon was almost 80% full with the terminator at 33.7 deg. west. In all images, North is to the top, Lunar West to the left.

In chronological order some images and captions:

The large crater to the left of centre is Copernicus. 93km wide and 3.8km deep. The terraced walls of the crater and the 3 central peaks are quite clear. The age of the crater is about 800MA, considered young and this has a ray system that is very prominent at Full Moon and clearly visible to the east,

Above Copernicus are Montes Carpatus, the southern boundary of Mare Imbrium.  To the right (east) is the crater Eratosthenes, the name holder for the Eratsthenian period, encompassing events with an age of 3000MA to 1000MA.

Although this is intended to be a Mare Imbrium tour, I had a short diversion to the Lunar South pole. I always struggle finding my way around the moon's southern polar area. Not too difficult for this period of the lunar cycle. The large crater at the lower centre edge of the photo is Clavius, 231km in diameter, but quite shallow at only 3.5km depth.

This is flanked to the west (left) by Scheiner (110 km dia and 4.5 km deep) and the large smooth lava filled crater to the NW, Longomontanus (145km dia and 4.5km deep).

Directly above Clavius is the famous crater Tycho (85km dia, 4,7km deep) which has a very visible ray system that can be seen unaided on a full moon. Typical of rayed craters, Tycho is considered young with an age of about 108MA, most craters are typically 3 to 4 billion years old.

Moving back to the north again but still pausing in the Southern Hemisphere. This is Montes Riphaeus and to the left, the small impact crater Euclides. This has a very similar form to most secondary craters. A secondary crater is formed from debris raised after an object collides with the moon. Secondary craters are typified by having a rounded bowl shaped depression with a raised and curved rim.

Montes Riphaeus forms the NW boundary of the small, lesser known sea, Mare Cognitum. The curved NE-SW trending wall of Montes Riphaeus is clarly the edge of an old crater about 300km in diameter that has subsequently filled with lava and then depressed to the SE. There are some small light coloured blemishes on the floor of Mare Cognitum. I originally thought that this was a blemish in the optics, but this does not appear to be the case. I need to do a little more research in this area,

Montes Riphaeus is also known as the running man and to appreciate this, you need to invert the image. To me, he looks like a footballer with spiky hair chesting a football!

The mountain range is Montes Appeninus (a peaking at 4600m high escarpment) starting from Eratosthenes crater to the WSW. This is the SW boundary of Mary Imbrium. The ray system from Copernicus is clearly visible across the dark mare floor. Near the northern end of Montes Appeninus is a small crater with is a bright western rim. This is Conon (24km dia., 2.4km deep).lTo the NNE of |Conon there is a small yellow circle. This is the approximate location of the Apollo 15 landing site. It really is remarkable how the pilots managed to land the Apollo lander safely and so close to the mountains. To the west of the landing site, there is a faint dark line. This is Rima Hadley, a rille which was subject to an investigation by the Apollo 15 lander crew. A rille is a graben, formed when the surrounding land is pulled apart (in this case, due to contraction as the lava cooled) and leaves a channel. This rille is about 1.2 km wide with a depth of  about 370m. A second, clearer rille is visible to the WNW of Conon, this is Rima Bradley.

The large crater to the top right is Archimedes, 81km wide and shallow (lava filled) at 2.1km deep. The depth of the lava has totally covered any central peaks that would have been visible. You can see there is zebra patterned ray material visible on the crater floor (tending E-W), the source of which is Autolycus crater (39km wide, 3.4km deep) located to the ENE.

To complete the picture, the smaller crater above Eratosthenes is Timocharis, 34km dia. and 3.1km deep. 

There is a slight gap between the last image and this one. This covers the North Imbrium basin, from Montes Caucasus and around to Sinus Iridum. The large rayed crater at the bottom/right of the image is Aristillus (55km dia., 3.6km deep). This has what appears to be a ring of central peaks, in reality its actually a cluster forming a loose cross. The rays are quite clear at this sun angle. To the top right of Aristillus is Thaetetus (25km dia., 2.8km deep). This has an unusual ployonal shape to the walls of the crater, possibly caused by slumping. The floor of the crater is flattened but not lava filled.

Montes Caucasus is a bit of an anomaly. The other mountain ranges circling Mare Imbrium follow a circular path. This is definitely offset, orientated NE and borders on Mare Serenitatis to the southeast.  Montes Alpes forms the north eastern perimeter of Mare Imbrium, with Mare Frigoris to the north. The slit through the Montes Alpes is the Vallis Alpes (Alpine Valley). The valley floor is flooded with lava. Despite its appearance of being sliced by a collision of some sort, its likely that this is a large graben caused by expansion and contraction as a result of the flooding of Mare Imbrium.

the large crater at the NW end of Montes Alpes is Plato (101km dia and a shallow 1.5km deep), clearly visible and famous for its very dark lava flooded floor. The crater floor is very smooth, punctuated by 5 small craters. I can see 4 of the craters (just) on the original high resolution image. On the Imbrium Basin floor, to the southwest of Plato there is a pair of mountain ranges, Montes Teneriffe and further to the west a larger range of mountains (Montes Recti) with a maximum height of 1.8km. 

Concluding this trip around the Imbrium Basin and still pre lunar dawn is the bay Sinus Iridum, flanked to the north and west by Montes Jura.

This pair of images, taken 3 hours apart show the sun rising over Sinus Iridum and the complexity of the formations of Montes Jura.

 

 

 

How to calculate the height of a lunar object from its shadow length

This is something that I have always wanted to do and I think this task is a very worthwhile exercise as apprentice Lunar observer.

On the 2025-01-09, we had a reasonably clear night and my favourite object, the Imbrium Basin was nicely illuminated. The moon was almost 80% full.

This image of the Northern part of Mare Imbrium was used as a basis for this example. The image was taken at 16:55 UT.

On the left hand (west) side, at the Northern end of the 'bay', Sinus Iridum, there is a distinctive triangular shadow that is due to the feature known as Promontorium Laplace. My task is to establish the height of this feature.

The image was cropped and measured as follows:

The large crater to the right (east) of the image is Plato, aged about 3.84MA. It is quite circular with an average diameter of 101km. We need to use this crater as a scale. Simply using a rule, the diameter of the crater was measured at 29mm across the peaks of the rim. Simple arithmetic (Crater Diameter/measured length) gives a scale of 3.48 km per mm.

Next thing to do is to measure the length of the shadow. This is measured from the highlight on the edge of the peak to the end of the shadow and I measured this at 11.5mm. Multiplying the 11.5mm by the scale value (3.48km/mm) gave me a shadow length of 40km.

 

We now have the length of the shadow. To calculate the height, we use the trigonometry formula Tan(sun angle) x Shadow Length.

We now need the Sun Angle. This can be read from almanacs, or, more simply in this world of computer systems there are other and probably more accurate ways of doing this.

This Sun Angle Calculator is an excel spreadsheet that will help you do just do that: https://the-moon.us/images/c/cc/Sun_Angle_Calculator.xls It's very simple to use, just enter the Selenographic co-ordinates of the Lunar Feature and co-ordinates of the Sub Solar Point

This website: https://www.internetsv.info/MoonCalc.html  will calculate the the Sub Solar point Longitude (Lo) and Latitude (Bo), in this case 56.3 E and -1.5 S

 

Note the unusual format for inputting the date and time. the date and time entries are separated by a . <full stop character> without spaces.

We enter the sub solar point co-ordinates onto the spreadsheet, exactly as displayed.

Next we need the selenographic co-ordinates of the feature, Promontorium Laplace. I use Wikipedia for this, I am  great fan and financial supporter of Wikipedia. Its not perfect, but it is certainly not bad. https://en.wikipedia.org/wiki/Promontorium_Laplace

OK, I can see that this also gives me the height as well, but that's not the point..

Enter the co-ordinates into the Lunar Feature box and the Sun Angle is displayed below: Note that you are entering Longitude as a negative number because it is west of the Central Meridian.

 

Sun Angle is 4.50 degrees.

From above, we can now calculate the height..

Height = Tan(sun angle) x Shadow Length.

Height = Tan(4.5) x Shadow Length

Height = 0.0787 x 40km

Height = 3.148km

Which is about 20% higher than the published figure of 2.6km.

Where did the error come from?

This is a very rough and ready method of calculating the height but there are some significant sources of error,

1. The measurements. It is very difficult to accurately measure the distances using a simple metric rule. The diameter of Plato is not easily defined, in this example, I measured across the rim which is apparently the correct way of measuring a crater diameter but the width of the rim and the sun angle makes the measurement points ambiguous. Additionally, the diameter quoted is the average diameter and several measurements across different aspects of the crater should be made and the result averaged. However, from the Earth viewpoint, the crater is oval shaped and this is due to foreshortening. You need to perform the measurement across the widest visible direction, i.e. East to West.

2. More measurements. I am trying to measure something that is approximately 40km long scaled from an object a few mm wide. It would have been much better if two separate images had been used (at the same magnification), a full frame higher resolution image of Plato and also an identically scaled image of the shade and the Promontorium.

3. Measuring directly with a rule on a screen is not ideal. From experience, draughtsmen (used to) use a pair of dividers to make a measurement and then transfer the spacing to a high precision rule. A more modern method would be to use a high end graphics program like 'Photoshop' or 'The Gimp' and measure the distances in pixels. 

4. I did not make any compensation in Longitude for any foreshortening, however, this would have made the apparent shadow length shorter.

I think this was still a very worthwhile exercise. I'll repeat it on some other objects where the depth or height has not been published.

Footnote:

https://the-moon.us/wiki/Promontorium_Laplace offers two different heights for Promontorium Laplace:

Clearly, pre Apollo era, others were having problems calculating the height as well.

A few other thoughts

To minimise errors, the shadow length needs to be as long as possible and across a flat surface (like a Mare).

In the top photograph, the crater to the upper left is La Condamine with a published diameter of 37km. Using the system described above, this gave a scaling factor of 1mm = 3.7km, and a shadow length of 42.55km..  This calculates the object height to nearly 3.5km which supports the thought that the image scales are too small.

Due to foreshortening, this system will only work for objects between 45W and 45E with errors increasing away from the central meridian. 

Conclusion

A worthwhile exercise. Providing that you accept that results may have a 20% error then this will give a very good indication of the height and depths of lunar features. The margin of error can be reduced by using a much larger image.

 

 

 

 

 

 

Jupiter - 20250109

A couple of images of Jupiter from 20250109.

Seeing Antoniadi 3. Very high level cloud and initially a very visible halo around the moon. Lots of condensation in the air which eventually limited the observing session as it condensed out and immediately froze. Hoar frost the following morning.

Very cold here with very average seeing, probably not helped by my C11 eventually freezing over. Eventually had to give up when ice started forming on the primary mirror.

Very strange conditions, the cold was quite bearable until about 21:00UTC when it suddenly became very much colder. I really need to get some warmer clothing for these conditions.

 

The first image was taken at 19:41 UTC. After processing, I was astounded by the vividness of the Great Red Spot. I don't believe that it was overprocessed, but it was almost invisible using an eyepiece at 80x magnification.

The moons that are visible are Io (left) and Ganymede (right).

About 40 minutes later (after the first major defrosting session on the telescope optics), I tried again but using a Baader modular Barlow (2.2x magnification). Results were a little disappointing. The GRS had moved further to the west and was now almost invisible, but I think the conditions were simply not good enough for this level of magnification.

Still fairly pleased with this though.

Central Meridian

Jovian Longitude (System III) for:

  • 2025-09-01 20:30 UTC
  • JD 2460685.354166666
  • 297.79 degrees

Optics

  • Celestron 11" Edge HD
  • Baader 2x Modular Barlow (one 2nd image)
  • ZWO ASI224MC Camera
  • IR/UV Block filter
  • Orion EQ-G Pro mount

Processing

  • SharpCap 4
  • AutoStakkert! 4
  • Registax 6

 

 

Mars - 20250102

In addition to the Jupiter images taken this evening, I eventually managed this image of Mars with some detail present.

Colour aberration probably induced by the x2 Barlow which is a shame.

Correlation of image details was against this excellent Albedo map on the BAA Mars section website by Martin Lewis

Martin Lewis Albedo Map of Mars

My own effort is a very poor rendition of this:

Seeing was Antoniadi III (at best) as Mars is still quite low in elevation.

Equipment

  • Celestron C11 Edge HD
  • Orion EQ-G Pro mount
  • Baader modular Barlow lens.
  • ZWO ASI 224MC Camera with IR and UV Block filter

Mars

Apparent visual magnitude:  Angular diameter (arcsec):   
Distance from Earth (a.u.):  Elongation from the Sun (°):   
Illumination (%):  Central-meridian longitude (°):   
Position angle of north pole (°):  Opposition 2020 countdown (days): 

 

Data from the Sky & Telescope Mars Profiler available from here

 

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