making astronomy accessible to all

Venus and Jupiter in daylight with an 8x20mm monocular
Venus and Jupiter are always much the brightest objects except the Moon in the night sky. In early July 2010 Venus was magnitude (mag) -4.1 and Jupiter -2.5. With the naked eye the former in the evening sky totally overwhelmed Mars mag +1.4, Saturn mag +1.1 and Spica in Virgo at mag +1.0 while the latter in the morning sky vastly outshone the surrounding stars in Pisces and the Square of Pegasus all of which are below mag +2.0.

Their brightness puts both planets in a class of their own and in the past they were generally the most used for sextant navigation* because their exceptional brilliance made them much more easily visible and identifiable at night. This was especially useful if the skies were too hazy to readily show the fainter navigational stars, mags +1.5 to +3.0 or too bright after sunset or before sunrise to show other stars at all. Indeed these two planets could even be used for navigation during the day.

As many of you will know, Venus when well away from the Sun can be seen with the naked eye alone in a good blue sky even when the Sun is high in the sky that is 30 degrees or more above the horizon**. Binoculars can help find it but here and I have to give this important warning: you must ensure that the Sun cannot be swept up by accident, ie:- it is hidden behind your house or other suitable screen.

The daytime sky can then be scanned safely without any of the modern aids now available to the amateur astronomer such as GOTO. I use 10x50 binoculars myself. Except when the planet is a crescent, Venus in the glasses looks star like but binoculars render the planet easily visible when it is beyond the reach of the naked eye alone. Sometime ago my son bought me an 8x20 monocular. This tiny optical appliance reveals the craters on the Moon and it sits easily in the palm of my hand with room to spare. It also shows Venus easily in the daytime when it is beyond the reach of the naked eye and the Sun is high in the sky. I proved this to myself on the 4th July 2010 when I saw Venus easily using my 10 x 50 Meade binoculars and the 8x 20 monocular at 11.10am BST (10.10am GMT.) The planet resembled a bright star like dot against the blue backdrop in both instruments. About one and a half hours later from 12.40 to 12.45pm BST (11.45am GMT) I could also (just) see the planet with unaided vision.

On previous occasions I have seen Jupiter with the naked eye in daytime when the Sun is 12 degrees or less above the horizon**. 10x50s show the planet as a small dull yellow disc which can be seen without difficulty in a blue sky, again when the Sun is high in the sky (30 degrees altitude or more.)

I was curious to see if my 8x20 monocular would show it and earlier that same day on the 4th July 2010 I had my chance. Jupiter was 7 degrees below the last quarter Moon and the weather was perfect. With the Moon as a guide I found Jupiter in my 10x50s but the planet was easily visible in the 8x20 monocular as an even smaller disc against the blue sky. Although invisible at all times with the naked eye I observed the planet off and on in the monocular from 7.25am BST (6.25am GMT) until 10.35am BST (9.35am GMT) by which time the Sun was 51 degrees high. Only the fact Jupiter was now too low and immersed in haze near the horizon prevented me following the planet for longer.

* References:
1) Astronomical Air Navigation Hadingham 1945
2) Reeds Nautical Almanac 1974

** I have found the sky brightness drops markedly leading up to sunset and increases by the same amount after sunrise but it is essentially unchanged when the Sun's horizon altitude is 30 degrees or more

Article by Peter Parish (July 2010)

Mid-Kent Astronomical Society
Website by and © Delta Consultancy Services