Phase of Venus on the 9th January 2025

Three days ago we had some more cold clear weather late in the afternoon. Venus is now quite a prominent object in the sky and seems very bright. It will get brighter still even though the illuminated area of the planet will be shrinking. This is because Venus is getting closer to us as it heads towards inferior conjunction. Maximum brightness will be around the 20th February when it reaches magnitude -4.9. On the 9th it was 1 day away from greatest elongation east and at about 26 degrees high in the SSW.

I was again out the front of the house with my 102mm Celestron:-

The most important thing about this observation was that in my opinion dichotomy (or half phase) has been reached! This was the feature that I had been waiting to see these last few weeks. I was again using an eyepiece and barlow lens combination that gave a magnification of 147x. When the image had settled enough (the breeze and seeing playing its part) I almost felt like I could detect some concavity in the shape of the illuminated side (which would mean that the phase was less than 50%). It definitely looked like half phase at the very least. This is against what would be expected for a solid sphere at Venus's current position. The predicted phase on this date is still 51.6% and it indicates to me that the phase anomaly (that dichotomy is observed about 4 days ahead of its prediction) is a real effect. Why this is so may be to do with the atmosphere of Venus and I will try and explain this another time.

One effect that I haven't been able to understand is that, to my eyes, the northern cusp still looked slightly rounded with the indentation of dark eating into the light side. This has been apparent all through this string of recent observations (the southern cusp on the other hand went straight to the edge with no deviation). I will be interested to see what happens a few days after the predicted date of dichotomy.

All text and images © Duncan Hale-Sutton 2025

Drawing of the crater Atlas on the Moon (3rd January 2025)

After I had completed my phase observation of Venus on the 3rd, I thought I would have a go at drawing some feature on the nearby Moon. The Moon at this stage was only 4 days old and a nice thin crescent. Near the terminator were a number of nicely lit craters and I settled on this one to draw:-

Atlas is an 89 km diameter impact crater that can be found at latitude 47 degrees North and longitude 44 degrees East (lunar coordinates). I was using my Celestron NexStar 102 SLT with a 9mm orthoscopic eyepiece and a star diagonal. This gave a magnification of 73x. I started observing at 16:25 UT and finished at 16:48. The seeing was pretty good and AII on the Antoniadi scale.

The first thing to point out was that I didn't know I was drawing Atlas before I started. I had to work this out after I had finished (I did draw the positions of craters Franklin and Cepheus as a guide to recognition). What amazed me was my lack of understanding of the orientation of what I was looking at! Fortunately, I have a very good Atlas (the Collins Atlas of the Night Sky by Storm Dunlop) which has lunar maps for each type of telescope combination. So with a refractor with a star diagonal in the position I had it in, lunar West and East were in the right position but North and South were flipped (see my diagram). This was a surprise to me and, as I had been using the same set-up for my Venus observations, it meant that I had been assuming North and South were the wrong way round (not so easy to tell this for a featureless surface like Venus). Ho hum, as they say. Rookie error! So if you want to view my drawing in the usual manner you would have to reflect in a line along the east-west axis.

Note also another very confusing thing that I didn't appreciate. When you look at the moon directly what is labelled as the East side of the Moon is not on the east side of the sky - it is on the west! It was decided to have this type of labelling because then the terminator moves from lunar East to West, a bit like on Earth. Very confusing if you are trying to orientate yourself.

Anyway, I was able to identify that it was Atlas that I had been drawing. Note that Atlas was just beyond the terminator and in the illuminated part of the Moon. West of Atlas the Moon was in shadow. You can see there is quite a dark wall on the North side of the crater and this because there is another crater next to it (Atlas E) which causes this feature (have a look at this Sky at Night article and the image of this area). I think another smaller crater (Atlas A) is to the South.

My drawing may look a bit small but in my defense I would say that drawing at the telescope in freezing conditions is not easy! This is the first time I had tried this in a serious way and it was quite taxing to try and get the details right.

All text and images © Duncan Hale-Sutton 2025

Phase of Venus on the 3rd January 2025

More clear weather was to be had two nights ago on the evening of the 3rd January and I was back at the telescope looking at Venus. At about 4pm Venus was now quite high in the sky at an altitude of 24 degrees and, as said in my previous post, only 3 to 4 degrees away from a 4 day old moon. It made a pretty conjunction. I again used my Celestron 102mm with a 9mm eyepiece and x2 Barlow lens. The seeing was again good and estimated to be AII on the Antoniadi scale. There was a bit of intermittent cloud. Here is the page out of my notebook:-

One new thing you will notice is that I have printed out a planetary blank (courtesy of Paul Abel) and stuck it in my notebook prior to observing. All I needed to do was shade in the dark half of the planet. I again estimated the phase by comparing what I saw at the telescope with a series of phase diagrams ranging from 50 to 60% in steps of 1%. My estimate this time was 53% and this is what this looks like when drawn on a computer:-

Looking at the data in the BAA Handbook (and interpolating) the phase was predicted to be 54.5% and so I had underestimated it by 1.5%. Here is what 55% looks like on a computer:-

There isn't a lot in it. There is a bit more curvature in the terminator and indetation at the 'poles'.

All text and images © Duncan Hale-Sutton 2025

Conjunction of the Moon and Venus (2nd January 2025)

The new year has got off well with a clearish beginning to the evening of the 2nd January. A three day old moon was now in the sky and in the same area as Venus. This made a pretty conjunction and so I got out my camera to take a picture:-

The photo was taken at 16:32 UT with my Nikon D90 set at a focal length of 50mm (1/5s at f/5, ISO 400). Some cloud was beginning to roll in as you can see and the clear sky didn't last that long! Actually, the Moon and Venus were even closer the following day, separated by perhaps 3 or 4 degrees.

All text and images © Duncan Hale-Sutton 2025

Another phase observation of Venus (29th December 2024)

Sunday night we were treated to a clear night and late in afternoon I dragged the telescope out again to have another look at the planet Venus. I had a couple of improvements to add to my observations this time and one of these was a new x2 Barlow lens made by Astro Essentials and purchased from First Light Optics. It seemed a reasonable quality lens for the price and has a T-thread which means I can attach the camera to the back of it. This will enable me to do some planetary work with my camera on my other scope (an Orion OMC-140 with a focal length of 2000mm). 

The other improvement was to have a series of diagrams of what an illuminated sphere looks like at phases of 50 through to 60% in steps of 1% (as mentioned in my previous blog entry). I uploaded these diagrams to my phone and then at the telescope I could compare them to the image of Venus. It was then a case of swiping from one to the next to try and match what I could see to what might be predicted. I think this worked quite well. Here is the page from my notebook:-

 

I started observing about 16:30 UT and finished at 17:04. The seeing was better this time and estimated to be AII (slight undulations with moments of calm lasting several seconds). Venus was in the SSW at a higher altitude of 19 degrees compared to five days ago. Note also that the magnification was now 147x leading to a much larger image.

By comparison to my phase diagrams I thought the phase was between 56 and 57% and my drawing reflects what I saw. I was still under the impression that the northern cusp was more prominent than the one in the south. The BAA handbook had the phase as 0.569 on this date in good agreement with what I saw. In actual fact the BAA lists the phase as 0.582 on the 26th December 2024 and 0.555 on the 1st January 2025 and I had to linearly interpolate.

Here is what 57% phase is supposed to look like:-

All text and images © Duncan Hale-Sutton 2025

Venus on the 24th December 2024

Further to my my previous post I had another view of Venus on Christmas Eve. The weather had been pretty overcast in the lead up to Christmas and so I was pleased to see the planet at all. In fact, I had an additional incentive to make this observation as the BAA had a challenge to observe something on Christmas Day (or alternatively, if the weather was poor, Christmas Eve or Boxing Day). As it turned out Christmas Day and Boxing Day were a wash out, so I am glad I got a clear (ish) window Christmas Eve.

Around 6pm GMT I could see Venus low down in the SW at an altitude of about 11 degrees. There was a lot of thin cloud or mist about, but that didn't matter too much as the planet was bright enough to be seen clearly. I started making another drawing of its phase about 18:04 UT and finished observing at 18:18. This time I made an estimate of the seeing which was AIII on the Antoniadi scale (AIII corresponds to moderate seeing with larger air tremors). Here is the page out of my notebook:-

 

I was again using my NexStar 102 SLT and a 9mm eyepiece. I think this has been a learning experience for me. This time I paid quite close attention to the shape of the terminator and you will see, comparing it to the drawing I made on the 14th, that the cusps of the terminator (where it meets the 'poles') are much more indented than on my earlier drawing. In fact, I made a note about this on my drawing as you can see (it seemed to me that this indented cusp was more pronounced in the north). Taking my drawing again as a measure of the phase, the terminator was found to be 17.5mm from the right edge and since I had drawn my circle to be the same 32 mm diameter as before, this meant that my phase estimate was 17.5/32 = 0.547. 

According to the BAA handbook it was predicted to be 0.590 and so this would mean I had underestimated the phase by 0.043. However, I think there has been a bit of an expectation on my part about what the shape of the illuminated half of Venus should look like. In actual fact, by paying attention to the shape of the cusps I have produced a drawing that is much more like what would be predicted for an illuminated sphere. To aid me in understanding what the predicted shape should look like I have produced a number of drawings on the computer ranging in phase from 50% to 60% in 1% intervals. Here is the one for 59%:-

I think my drawing for the 24th December is accurate in shape for the cusps but is too linear near the equator. This may have been an expectation on my part that the terminator should be closer to the 50% mark than in my drawing on the 14th. In conclusion, it seems to me that perhaps I was underestimating the phase on these two dates.

All text and images © Duncan Hale-Sutton 2025

The phase of Venus 14th December 2024

I have been wanting to get out for a week or two now and see Venus with my telescope since this planet has been more visible in the evening sky. Venus is approaching its greatest elongation east on the 10th January 2025 and it will then be at its furthest angular distance from the sun as viewed from earth. However, this is not going to be a very good apparition because the ecliptic (near to which Venus travels) is going to be low down in our sky.

One of the things I wanted to do with an observation is to estimate Venus's phase as it approaches elongation. When Venus came out of superior conjunction on the 4th June, its phase was full and ever since then its phase has been decreasing and it will reach dichotomy (half phase - equal dark and light) on January 12th 2025. On the 14th December I spotted Venus low in the SSW at an altitude of 5 to 6 degrees. Although there was a bit of intermittent thin cloud I thought it worth getting out my Celestron NexStar 102 SLT and having a look. I didn't bother with a proper set up of this computer controlled telescope but just pointed it in the right direction. Here is a page out of my notebook:-

I used a 9mm Orthoscopic eyepiece that I have owned for about 43 years to get a good view of the planet. The NexStar has a focal length of 660mm and so this gives a magnification of about 660/9 = 73. I had drawn a circle in pencil in my notebook and then at the telescope at around 17.10 UT I drew the phase as I saw it. I had no preconceived notion of what the phase would be and so my choice of shading was as I thought it should be.

I should first point out that the phase is technically the ratio of the apparent area illuminated to the total apparent area of the planet. This is not very easy to measure directly! However, provided that Venus behaves like an illuminated sphere then the phase can be determinded from measuring how far along the "equator" the terminator is and then dividing this by the apparent diameter. I will prove this at some point in another blog entry.

From my diagram the terminator was 18 mm from the right edge and the diameter was 32mm and so I estimated the phase to be 18/32 = 0.563. The predicted phase on this date from my BAA handbook was 0.631 and so the difference was 0.068. Note that with my use of a star diagonal south is at the top of my image but west is still to the right.

I don't think this was too bad. In actual fact many observers underestimate the phase of Venus in an eastern elongation and this is known as the Schröter Effect. I will try and write about this more later.

All text and images © Duncan Hale-Sutton 2024