Now is the time to see Venus and Mercury! Venus is at greatest elongation on Saturday, January 8 and Mercury is at greatest elongation on Sunday, January 9.
What is greatest elongation? Greatest elongation occurs when the angle between the Sun and either Mercury or Venus is largest as seen from Earth. The time around greatest elongation is a good time to view these planets as they are near their highest points above the horizon.
Mercury is so close to the Sun that even the largest angle is still small, (between 18 and 28 degrees). This is why it is only visible during the evening or morning twilight. On January 9, Mercury will be 23 degrees west of the Sun. In contrast, Venus will be 47 degrees west of the Sun on January 8 (about twice the angle of Mercury). Because they are both west of the Sun, they rise before the Sun and are visible in the early morning sky. Look for them in the southeast before sunrise. Venus will appear as the brightest object in the sky. Mercury will be very faint, much closer to the horizon.
Mercury moves quickly, so it will disappear from the morning sky by the end of January. It will reappear in the evening sky by mid-March and will be at greatest elongation east of the Sun on March 22, 2011. This will be the best opportunity for northern hemisphere observers to view this planet in the evening this year. Look for it almost due west about 30 minutes after sunset.
Venus remains in the morning sky until early July. It will return to the evening sky in late September.