So You Want To Take a Sight on Venus Tonight...

We will ignore the obvious with Venus being close to it's brightest these days at magnitude -4.8, so it will not be hard to find.

The following assumptions will apply...
- artificial horizon
- twilight about 40 minutes, centred on 1800 EST
- Freiberger sextant with IE on the arc = -3.0'
- DR of N 43 deg and W 079 deg

We will do it without technology aids to determine where and when to look with a preset sextant altitude.

The azimuth Z and computed altitude Hc will be close because we are doing it the longform way with tables that are based on whole number entry points. But this will be enough for us to catch Venus within the scope of the sextant when pointed in the right direction.

Don't forget that Z is true and you will need to adjust if using a hand bearing compass for variation - here at Toronto = W 011 deg.

The worksheet is completed just as it would be done for a typical sight, just no actual Ho to compare to, yet.

So you can see from the above worksheet that if you point your sextant towards 238 deg T (249 deg M) at 1800 EST you should pickup Venus at a preset altitude of 37 deg 39.0'

What do we get by looking ahead on Stellarium...

I get...azimuth 237 deg/altitude 35 deg 30.0'

The difference comes about because of the whole number Lat of N 43 deg and our AP Long of W 079 deg 12.9' versus actual of N 43 deg 43.0 and W 079 deg 24.0. We will be close enough to pick it up in the sextant.

So, if the skies remain clear tonight, we will see...

Here is the worksheet from the actual sight and plot...

You can see that Venus was where it was expected to be - any surprise? I took the sight about 10 minutes after 1800 because I almost forgot, but sprang into action as quickly as I could. Venus is going down fast so the altitude is less and also because I am actually further North and further West from the whole number values of the tables.

On the plot you can see that the LOP of the sight reduction is within 1 nm of Actual - this is because the optics through the sextant are a little tricky, there is at times a doubling up of the planet image through the scope (my failing eyesight? I don't use glasses). But this kind of sight if taken on the real ocean would be bang on.

Besides Venus, you can take this approach to pre-locate any other of Mars, Jupiter and Saturn - as well as any of the navigational stars - albeit with a slightly different worksheet approach that involves using the SHA and declination of individual stars