Venus and Sirius – 2 Bright Stars in the Evening Sky

First a meteor update: The weather continues to be bad for meteor observing in both Tucson and San Diego. Though my camera has been able to pick up a meteor or two per night between the clouds, daily updates don’t make for very exciting reading.

There have been quite a few comments on this site regarding one or two bright stars that are visible every night in the evening. The two stars in question are Venus and Sirius.

By far the brightest “star” in the sky during the evening hours is Venus. Though to the eye it looks like a star, Venus is actually the 2nd planet from the Sun. For the next few weeks Venus is at its brightest and highest in the sky. This combination makes it the most obvious object in the sky after the Sun and Moon.

Venus appear as a brilliant yellow star many times brighter than any other star in the sky. It is ~18 times brighter than the brightest star Sirius (located in the southeast) and ~75 times brighter than Capella (the bright star located nearly over head in the evening). It is so bright in fact, that it can be fairly easy to see in broad daylight if one knows exactly where to look. Located 40 degrees above the southwest horizon at sunset, it sets below the horizon around 9 pm.

Why is Venus so bright? It is due to a combination of factors. Venus is covered in a permanent shroud of highly reflective clouds. These clouds reflect ~65% of the sunlight that hits it. For comparison, the Earth reflects ~35% and Mars and the Moon around 13-15% of the sunlight striking them. Venus is also rather large, being only a bit smaller than the Earth (its radius is ~95% as large as Earth’s). The final piece of Venus‘ brightness puzzle is its close distance to Earth. Right now Venus is 0.494 AU (~46 million miles) from Earth. By comparison Saturn, the other bright planet visible in the sky, is 8.50 AU (~790 million miles away). Only Mars can get as close though it can never get as bright because it is smaller and less reflective than Venus.

Through a telescope or pair of good binoculars Venus appears like a miniature crescent Moon (see the image below). Over the next month and a half, Venus will appear larger and larger. It’s crescent shape will also become more narrow. If I can get my telescope and camera system to work properly, I will try to post some Venus images over the next few weeks.

Predicted telescopic view of Venus for February 10, 2009. Image from the NASA JPL Solar System Simulator. (http://space.jpl.nasa.gov/)

The 2nd bright “star” in the evening sky is Sirius. Sirius is the brightest star visible in the night sky with only Venus, Jupiter, and sometimes Mars being brighter. By the time it gets dark, Sirius is visible low in the southeast. It is visible for most of the night in the southern sky.

Sirius is a blue star but can appear to change color rapidly. The reason for this is due to the Earth’s atmosphere. Turbulence in the atmosphere causes the star’s light to be “bounced” all over the place. The light of the star is made up of many different colors which all “bounce” around differently. As a result, normally blue Sirius can appear to rapidly switch between many different colors when it is close to the horizon (meaning its light is passing through more atmosphere than usual). All stars experience this effect, it is just that Sirius‘ brightness makes it more evident. Watching Sirius when low in the sky with a telescope or just your eyes can be one of the best sights in the night sky.

Why does Sirius twinkle and change colors while much brighter Venus does not? Check out Phil Plait’s explanation on his Bad Astronomy site.

More on Sirius can be found here.

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