Venus reaches inferior conjunction on 13 August, when at 12:15 BST (11:15 UT) it will pass 7.7° from the centre of the Sun.
This is an inferior conjunction, the moment when Venus lines up with the Sun on the Earth side of its orbit.
The alignment isn’t precise and in 2023 Venus ends up passing south of the Sun.
Some inferior conjunctions are closer than others.
Venus’s changing appearance as it orbits; it’s now approaching 50% lit (dichotomy). Credit: Pete Lawrence
The inferior conjunction on 13 August brings Venus to 7.7° from the Sun’s centre at 12:15 BST (11:15 UT) and with great care and pre-existing knowledge of how to observe such an event, Venus can still be picked up using a camera.
The dangers of observing this close to the Sun are obvious and it’s important not to push your limits if you’re not sure what you’re doing.
Pointing a telescope this close to the Sun runs the risk of light passing down the optical tube.
If this happens, the scope may be damaged. You should certainly not attempt to view this close to the Sun visually, only via a camera with a live feed.
Observing Venus in August
Images from 2007 showing an ultra-thin crescent Venus passing south of the Sun. Credit: Pete Lawrence
On 1 August, Venus sits 18.6° from the Sun but isn’t visible in the evening sky after sunset.
Therefore it’s necessary to hone your skills for locating the planet during the day (see below for more on how to go about this).
On this date, Venus shows as a slender 5%-lit crescent, 53 arcseconds across.
One week later on 8 August the separation from the Sun will have decreased to 10.4°, Venus showing as a very delicate 1%-lit crescent with an apparent diameter of 57 arcseconds.
At inferior conjunction on 13 August, Venus will be showing a phase of 0.9%, now appearing 58 arcseconds across.
An interesting phenomenon occurs when Venus’s separation from the Sun decreases below 12°, the horns of the planet’s delicate crescent appearing to extend further than they should to form what are known as cusp extensions.
Record Venus at inferior conjunction
A telescope's setting circle
With care you can make a record of Venus as it approaches inferior conjunction.
First and foremost, pointing a telescope close to the Sun is dangerous and extreme care needs to be exercised.
If you’re not sure what you’re doing, we strongly recommend that you don’t attempt this.
The safest setup to use would be a small to mid-sized refractor along with a high-frame-rate camera fitted with a red or infrared pass filter.
Don’t attempt to see this visually.
Any finderscopes will also need to be capped or removed.
You’ll need a full-aperture white-light solar filter and for the purposes of this challenge we assume that you’ll use a driven, polar-aligned equatorial mount which has setting circles.
Setting circles aren’t complicated to use, but you need to pay attention to the values to make sure you’re using the correct scale
Set the telescope up and turn on the mount drive.
A wide camera field is recommended, ideally one that sees the entire Sun in one go.
Remove or cap finders.
Check that the solar safety filter isn’t ripped or lets light through – discard it and use a new one if so.
Once checked, fit it and point the telescope at the Sun.
Focus as accurately as possible on the Sun’s disc.
Once focused, centre the Sun in your field of view using a crosshair overlay, if your camera’s control program has this option.
Otherwise, use your best judgment. Set the mount’s slew speed slow enough that it takes several seconds for the Sun to cross the field of view.
Use a planetarium app (for example Cartes du Ciel) and work out the Sun’s right ascension (RA) and declination (dec.) for the time you’re observing.
Unlock the dec. setting circle scale and rotate it so it reads the Sun’s dec., then lock it again.
Make sure you’re using the part of the scale that increases in value if you move the telescope north.
Similarly, set the setting circles to the Sun’s current RA value.
Use the scale that increases if you move the scope east.
Once set, look up the RA and dec. coordinates of Venus.
Carefully move the scope so the setting circle pointers indicate the values for Venus.
As a safety check, ensure you’re not pointing at the Sun, then carefully remove the solar filter.
Double-check that sunlight isn’t passing too far down the tube – if it is, re-cover the scope and abort the attempt.
Adjust the camera settings to boost the sky brightness and look for the fine, 0.9%-lit Venusian crescent.
If it’s not visible, you can apply short slew bursts to emulate a spiral pattern in the sky.
Be careful though – you don’t want to slew too far so that the Sun appears!
If you simply can’t find the planet, replace the solar filter and start over.
This guide originally appeared in the August 2023 issue of BBC Sky at Night Magazine.
