Skywatching Tips: Observing Venus Without Staring at the Sun

Venus is currently a prime observing target, but skywatchers should be careful aiming telescopes at the brilliant planet, because it is appearing close to the sun at dusk.

Venus? orbit is closer to the sun than Earth?s, so the planet passes between Earth and sun once every orbit. This is known as inferior conjunction (to distinguish it from when Venus passes on the far side of the sun, known as superior conjunction).

This sky map shows where to look to spot Venus this week.

Venus was at its greatest brilliancy Sept. 23, and is now moving swiftly toward inferior conjunction Oct. 28.

Since the orbit of Venus is not in exactly the same plane as that of Earth, it usually passes above or below the sun at inferior conjunction. This year it will be passing below the sun.

In addition, at this time of year the plane of the ecliptic is almost parallel to the horizon at sunset. As a result, Venus will be very low in the western sky at sunset, and hence very hard to observe. [Photos of Venus crossing the sun.]

Skywatcher warning

The trick for observing Venus under these conditions is not to wait until sunset, but to observe the planet while the sun is still above the horizon.

But first, a skywatcher warning: Observing objects while they are close to the sun is very dangerous! This is something only very experienced observers should attempt, and they should take careful precautions to avoid viewing the sun directly.

The trick to use is to position yourself so that the sun is safely behind a nearby building or other obstruction, so that there is no way that you can see the sun itself from your location.

In the accompanying sky map, we?ve used one of the stones at Stonehenge to block the sun; you can probably do the same with a less glamorous chimney or tower.

Before observing with the naked eye, you may find sweeping with binoculars necessary to initially locate Venus. Use a planetarium program to determine the direction and distance of Venus relative to the sun.

A better technique is to use a telescope equipped with either accurate setting circles or a GoTo computer.

Once located, Venus is readily visible in a clear blue daylight sky.

Venus in October sky

Today (Oct. 6) Venus appears like a miniature crescent moon 14 percent illuminated. It will appear to be 49 arc seconds in diameter, large enough that its crescent can be seen in 10x binoculars.

An arc second is a unit of measurement for objects in the night sky. There are 60 arc seconds in 1 arc minute. The full moon, for example, is about 1,800 arc seconds across.

It is slightly more than 30 degrees away from the sun, and quite safe to observe.? For comparison, the width of your fist held at arm's length covers about 10 degrees of the sky.

Over the next few weeks Venus moves south and west so that it is directly under the sun Oct. 28, only 1 percent illuminated and a full arc minute in diameter. Only 6 degrees separate Venus from the sun.

Since this is less than the width of a binocular field, it is unsafe to attempt to observe Venus this close to conjunction.

The question remains, how close to conjunction can you observe Venus safely?

On Oct. 20, Venus will be 15 degrees away from the sun, probably as close as even the most experienced and careful observer would want to get. Don?t risk your eyesight by trying to observe Venus any closer to the sun than this.

Instead, wait until the next inferior conjunction of Venus ? June 5, 2012, when Venus will actually pass in front of the sun, and can easily and safely be observed with a standard solar filter.

• Gallery - Venus Seen From Around the World
• Telescopes for Beginners
• Telescopes Up! A Guide to the Night Sky's New Stargazing Season

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.