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

Recent variable star observations (23rd October, 25th and 29th November)

I have been carrying on with my variable star observations (these are mostly pulsating variables). The evening of the 23rd October was one day before last quarter and the Moon wouldn't rise until 21:11 UT. Astronomical twilight ended at about 18:40 UT. The skies were clear but the transparency wasn't great.

Here are my observations:-

TX Dra, 19:25 UT, chart 106.04, S(2)V(1)K, mag.6.9

AH Dra, 19:40 UT, chart 106.04, 2(3)V(2)6, mag. 7.6

CH Cyg, 20:08 UT, chart 089.04, A(1)V(3)W, mag. 6.7

Z Uma, 20:33 UT, chart 217.02, =C, mag.7.5

On the night of the 25th November, a month later, the Moon was 2 days past last quarter and wouldn't rise until 02:21 UT. Astronomical twilight ended at about 18:00 UT. The sky transparency was average. One of the mistakes I made was to assume that it was still British Summer Time and so I recorded my observation times as an hour earlier than they should have been. Fortunately, I saw my mistake and corrected it. Here are the observations:-

CH Cyg, 18:40 UT, chart 089.04, =A, mag. 6.5

TX Dra, 19:07 UT, chart 106.04, =N, mag. 7.7

AH Dra, 19:15 UT, chart 106.04, =6, mag. 7.8

RW Cep, 22:17 UT, chart 312.02, B(1)V(3)E, mag. 6.7

W Cep, 22:23 UT, chart 312.02, D(4)V(1)F, mag. 7.4

GO Peg, 22:50 UT, chart 103.02, B(3)V(1)E, mag. 7.6

Z Uma, 23:21 UT, chart 217.02, =B, mag. 7.3

A few nights ago on the 29th November it was 2 days before new moon. Astronomical twilight ended about 18:00 UT and the sky transparency was good. Before this observing session I was looking for some new variable stars to observe to add to my roster. The ones in Cygnus below were the first of these. I also had a go at AC Herculis which I hadn't observed since 2023.

AC Her, 19:19 UT, chart 048.4, D(1)V(1)E, mag. 7.8

AF Cyg, 21:51 UT, chart 232.02, =Q, mag. 7.6

V973 Cyg, 22:02 UT, chart 232.02, =F, mag. 6.7

All seen with 10x50 binoculars.

All text and images © Duncan Hale-Sutton 2024