Watching Venus
Filters and a logbook will help you track the planet’s changing face
People often find their first telescopic view of Venus disappointing, as they tend to observe it when the sky is darker and the planet is very brilliant. At such times, Venus is low down. The glare from the disc and poor seeing mean all you are likely to see is something resembling a steam pudding! The best time to view Venus is during dusk, when the sky is still bright and the planet is higher in the sky.
You will need at least a 3-inch telescope and a magnification of about x120 to see any details beyond the phase. Give your eyes a chance to get used to looking at Venus, then see if you can make out the bright areas covering the north and south poles, called cusp caps. They are often bordered by darker grey cusp collars and all of these are likely to change from night to night during the elongation.
Sometimes the cloud markings present a distinctive Y-shaped feature and bright spots may also appear, often close to the limb – although bear in mind that seeing cloud markings on Venus is easier for people whose eyesight is more sensitive to blue light.
Another phenomenon associated with Venus is the Schröter effect, when the observed phases of the planet are different from the predicted phases (see below). This will be obvious in small telescopes. You will also find using optical filters a great help when observing Venus. Most telescopes come with a set. They are easy to use and usually screw into the barrel of the eyepiece. On the side of each filter you’ll find printed its Wratten number and this uniquely identifies it. For example, a yellow W12 or W15 filter will help sharpen Venus’s features and reduce the glare, while a blue filter like a W38A or W47 can help to bring out cloud markings. You’ll find cloud markings and features change appearance in different filters, as they reveal different levels of the atmosphere.
You will get more out of your observing sessions if you write down what you have seen. I now have several Venus logbooks going back decades, where I record the combination of eyepieces and filters that worked best, and from the drawings I can see which phenomena repeat themselves. A record like this means you are not returning to the eyepiece new each time. Instead you’re actively building on your experiences. I find it invaluable.
Making a drawing is the best way to record your observations. I always use a 50mm blank and start by drawing in the phase, then putting in any cloud markings and the cusp caps and collars. Make sure that you record the date, time (in UT), the telescope you used and magnification. If you have made a drawing using a filter, make sure to record its number and details. Over the course of an elongation, you will see your drawings improve and the amount of fine detail you can see will increase.