How to see Venus in daylight in 2012

posted: 1808 days ago, on Sunday, 2012 Aug 12 at 14:47
tags: astronomy, Southern Sky Highlights 2012, Sun, Moon, Venus.

Brilliant Venus, doing double-duty as Evening Star and Morning Star as its orbit around the Sun carries it into our dusk and dawn skies, is the brightest planet we can see from Earth. In fact, it is ten times brighter than Sirius, the brightest star in the night sky.

Venus is usually brighter than magnitude -4, and can be seen during broad daylight. In Xhosa tradition, a young boy would have to prove that he can see iKhwezi Iesibini (Venus) in the daytime in order to be accepted into the circle of true shepherds.

For any object to be visible with the human eye, it needs to be (a) large enough, and (b) stand out against whatever background it is being viewed on.

In magic, for instance, "black art" is the cunning use of a black object on a black background, which effectively renders the object invisible. A thin black thread attached to a rose, for example, could create the illusion of the flower floating freely yet somehow under the mystical control of the magician.

The visual contrast of an object, Cv, can be defined as

Cv = (Ib - Io)/Ib [1]


Ib = intensity of the background,

Io = intensity of the object.

Experiments suggest that a Cv ratio of 0.02 (or, 2%) is the threshold of human perception under normal daylight conditions.

Astronomically speaking, the intensities Ib and Io can be approximated by surface brightness, expressed in magnitudes per square arcsecond (mpas2). By this measure, the Sun weighs in at -10.7 mpas2; each square arcsecond of the Sun shines at magnitude -10.7. A pristine dark night sky only manages +22.0 mpas2. The clear daytime sky, at the horizon, has a surface brightness of about +3.0 mpas2. With the Sun just 5 above the horizon, the sky at the zenith darkens to +6.5 mpas2.

Now consider Venus. At maximum elongation, Venus has a surface brightness of +1.9 mpas2. Plugging this value into equation 1 gives for Venus a Cv = 0.37 for Venus in normal daylight sky, substantially greater than the Cv limit of 0.02 and confirming what we know - Venus is bright enough to be seen in the daytime sky.

The problem with seeing Venus during the day lies simply in finding it. Knowing where to look is the first step. Against a featureless blue sky, it's anyone's guess what your eye is focusing on in any case.

From time-to-time, the happy event of Venus near the Moon during the day crosses one's path. The Moon is both bright and large enough to be seen during the day, and acts as a visual anchor for spotting Venus. It's also a great focus aid if you want to cheat and first use binoculars.

The diagrams below show the dates during 2012 when the Moon is near Venus. They are drawn for 12:00 SAST, but since the angular separation between Venus and the Moon doesn't change much during the course of the day they can be used earlier or later.

As a footnote, we can do a similar calculation for the visibility of Jupiter during the day. Jupiter has a surface brightness of 5.7 mpas2, which suggests it is not visible in broad daylight (+3.0 mpas2) but can be seen when the Sun is 5 above the horizon (+6.5 mpas2) - in other words, soon after sunrise or just before sunset.

Finder chart for seeing Venus in the day during 2012

Find out more: Southern Sky Highlights for 2012

nothing more to see. please move along.