The Unusual Path of Venus

Venus' southern hemisphere, as seen in the ultraviolet. CREDIT: ESA

Later this month the planet Venus will do the unusual, remaining visible throughout the time it sweeps between us and the sun.

Normally this brilliant planet becomes lost in the solar glare as goes through what's called inferior conjunction. This time, however, the tilt of its orbit (3.39-degrees from Earth's) will carry Venus widely to the north of the sun from our point of view. So, for a few days around conjunction it will be possible to glimpse the planet both as an "evening star" low in the west after sunset and as a "morning star" in the east before sunup.

The setup causes Venus to go through phases, much like our moon.

At its biggest, the slender crescent of Venus will measure nearly 1/30 the apparent diameter of the moon, large enough to resolve with 7-power binoculars and perhaps even with the naked eye by those who possess exceptional visual acuity. The horns or cusps of the crescent will always point away from the sun, and viewers will see them extending upward (toward the north) while Venus swings past conjunction.

March 25: Morning and Evening Parity

Seen from mid-northern latitudes at this time of year, the ecliptic – the imaginary line across the sky representing the path of the sun – is steeply inclined to the horizon in the early evening. Therefore, Venus appears to descend the western sky rapidly during these last couple of weeks of March.

On March 25 at 14 hours Universal Time the sun and Venus are in conjunction in right ascension with the planet 9.1-degrees due north of the center of the sun's disk. On this date viewers in North America will see Venus about equally well in both the evening and morning sky, almost 9-degrees to the upper right of the setting sun (your clenched fist measures roughly 10-degrees at arm's length) and about as far to the left of the rising sun. From latitude 40-degrees north (the latitude of Philadelphia, Denver or Madrid), Venus will be 5-degrees above the horizon at both sunrise and sunset!

Up to and including the evening of March 25, Venus is easier to see in the evening sky, and thereafter easier in the morning.

Sighting the Slender Sliver

Venus passes inferior conjunction on March 27 at 19 hours Universal Time, when its celestial longitude (position along the ecliptic) is the same as the sun's. Viewed from a point above the solar system, the planet would appear directly in line with the sun and Earth. Actually Venus lies well north of the ecliptic plane at this time; we will see it 8.2-degrees north of the center of the sun's disk.

Although thinned to less than one-percent of the planet's diameter, the illuminated crescent will still be observable with slight optical aid. Using binoculars, try to observe Venus immediately after the sun goes down on March 27, by scanning the horizon about 7 or 8-degrees to the right of the sunset point and about 2-degrees higher.

You might even try looking for Venus during the daytime, but if you attempt this challenging observation, do not sweep for it with binoculars or a telescope, as serious damage to your eyes can result if the full blaze of the sun is accidentally encountered!

Probably the best and safest technique is to point your telescope the evening before at a star having nearly the same declination as Venus. Record the time, and then leave your telescope stationary to allow the rotation of the Earth to bring Venus into your telescope's view on the following day. The 3rd-magnitude star Vindemiatrix, in northern Virgo (also known as Epsilon Virginis) will be ideal for this purpose on March 24, 25 and 26.

So, the night before:

• Center your telescope on this star
• Note the exact time.
• Do not touch the telescope.

Venus will glide through the same field of view 11hr 17min later on March 24, 11hr 15min later on March 25 and 11hr 13 min later on March 26 (as seen around midday in North America). Of course, a low-power wide-field eyepiece is preferable. In addition, prefocusing your telescope will aid greatly in locating Venus by day. It also will help if the sun is hidden behind some obstruction like the roof of a house so that you and your telescope are in shadow with no part of the scope sunlit. As a rule, a very clear sky is required to see the faint extensions of Venus' cusps that make the crescent longer than a semicircle.

First Morning Appearance?

So on what date will it first be possible to sight Venus as a "morning star"? We can refer to a nearly identical inferior conjunction of this planet almost exactly eight years ago.

That year Venus could be glimpsed (with binoculars) in the morning sky nearly five days before inferior conjunction! Any sighting by March 22 or 23 of this year will equal that 2001 feat. However, the planet will not be an easy target. On March 23 it's about 10-degrees to the left of the rising sun but only 4-degrees higher (as seen from latitude 40-degrees north). The ecliptic makes a shallow angle with the morning horizon, so Venus gains altitude very slowly with each passing morning. From latitude 40-degrees north it comes up ahead of the sun as early as March 18 and from more northerly locations even sooner.

By March 30 Venus will stand almost 9-degrees directly above the rising sun, thereafter moving ever farther to the upper right in the twilight sky.

And from then on through the balance of this year, Venus will be purely a planet of the dawn.

• Video: What Happens to Venus
• Online Sky Maps and More
• Sky Calendar & Moon Phases

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.