After the Sun and Moon, the brightest object that makes a regular appearance in our skies is the planet Venus. It isn't always visible, but this month (March 2015) you can locate it in the western sky after sunset.
In some ways, Venus is similar to the Earth, and it is the Earth's closest planetary neighbor. Among the planets in our solar system, Earth is the third closest to the sun, and it orbits at an average distance of 93 million miles. This distance is called one astronomical unit (1 AU). Venus is the second closest planet to the Sun, orbiting at a distance of about 67 million miles (0.72 AU).
Venus is nearly the same size as the earth. If it were possible to shrink the Earth to a ball with a six-inch diameter and to shrink Venus using the same scale, Venus would have a diameter of five and three-quarters inches. Like the Earth, Venus is a rocky planet. That means its surface is comprised mostly of silicate rocks and metals rather than gasses.
Conditions on the surface of Venus are very different from those on Earth, however. Its average surface temperature is 730 Kelvin (which equals about 854 degrees on the Fahrenheit scale) — more than sufficiently hot to melt lead. It may be tempting to think that Venus is hotter than Earth because it is closer to the Sun, but this isn't the reason. Venus is hot because the planet's atmosphere (mostly carbon dioxide and sulfuric acid) works like a greenhouse to trap the Sun's energy.
Venus and Earth differ in other ways. The Earth rotates on its axis once every 24 hours (a time period called a day), and an Earth year takes approximately 365¼ days. Measured in Earth days, a year on Venus is 225 days. But Venus rotates on its axis much more slowly. A day on Venus lasts the equivalent of 243 Earth days. As these numbers suggest, a day on Venus is longer than a year on Venus. Furthermore, compared with other planets in the solar system, Venus rotates backwards.
If you would like to observe Venus this month, look for it in the evening sky just after sunset. You may see a dimmer, reddish object nearby. That's the planet Mars. Venus will appear low, near the horizon, during the early part of March. As the month progresses it will climb higher and increase its separation from Mars. If you get a chance to see it on multiple occasions, watch how its position changes in relationship to the stars. On March 16, Venus will cross from the constellation Pisces into Aries.
Venus and Mars put on a special show during the days that follow the new moon of March 20. On March 21, Mars will be next to the one-day old crescent moon. The next night, March 22, Venus will be beside the two-day old crescent moon. On March 23, Mars, Venus, and the moon will appear in a line. If you start with Mars and extend that line across the sky toward the East, you'll come to another especially bright object. That's the planet Jupiter. We'll talk more about Jupiter next month.