During December when you looked to the southwest at sunset, you saw a bright star about 20 to 30 degrees above the horizon. It wasn’t really a star, but the planet Venus.
During the month the planet moved closer and closer to the horizon each night. Now, at the end of the month, it is only about 15 degrees above the horizon and moving even more rapidly toward the sun.
By the middle of January Venus will disappear in the glare of the sun, and then, by month’s end, reappear in the morning sky before dawn. In February it will move farther from the sun, rising earlier each morning and appearing higher in the sky at dawn.
Why is Venus moving from the evening sky into the morning sky?
Both Venus and Earth have elliptical orbits around the sun, with Venus’s being closer to the sun.
With its closer orbit, Venus circles the sun in about 225 Earth days, while the Earth takes 365 days to complete one orbit.
This means that every 584 days (1.6 years), Venus passes the Earth and is on a line between the Earth and the sun. Astronomers call this alignment an “inferior solar conjunction.” This alignment will occur on Jan. 11, 2014, at 4:25 a.m. PST. The last time this happened was on June 6, 2012, and after Jan. 11 it will not happen again until Aug. 15, 2015.
From Earth, Venus will appear to pass in front of the near side of the sun, missing it by just a little more than 6 degrees. It is this apparent passing that causes our view of Venus to move from the evening sky to the morning sky.
At inferior conjunction the Earth and Venus are also the closest to each other, on average just 25 million miles apart. This makes Venus the closest planet to pass the Earth.
But on Jan. 11 the planets will be almost a million miles closer together than average because on Jan. 4 the Earth is at perihelion, the point in its orbit where it is closest to the Sun. (The word perihelion is from the Greek words peri, meaning near, and helios, meaning sun.) Venus’s orbit is nearly circular, so Earth’s perihelion is the closest the orbits come to each other.
Because Venus passes so close to the Earth, it will have its largest apparent size and greatest brightness in the weeks just before and after conjunction.
At magnitude minus 4.9, it will be the second brightest object in the night sky.
Since the orbit of Venus is inside the orbit of Earth, with a telescope we see the planet in a sequence of progressive lighting, giving the planet phases like those of the moon.
At inferior conjunction Venus appears in the new phase where we are seeing the night side of the planet. It shows a quarter phase when it is at its maximum elongation, its farthest distance from the sun, about 47 degrees. The full phase is reached when Venus is at superior conjunction, when the planet is opposite the Earth on the other side of the sun.
It takes 584 earth days to complete one cycle of the four phases. Galileo Galilei was the first person to record the phases of Venus, from his telescopic observations made in 1610.
Because Venus could only show phases if it were orbiting the sun, these observations essentially ruled out the Earth-centered Ptolemaic system and supported the sun-centered Copernican system.
At inferior conjunction, Venus appears to pass above or below the sun because its orbit is tilted by 3.4 degrees from the orbit of the Earth.
But if the conjunction occurs where the orbits cross, Venus will transit the surface of the sun. These transits occur in pairs every 243 years, with the pair separated by eight years.
The last transit occurred on June 5-6, 2012, eight years after its partner transit in 2004.
By chance, another planetary alignment occurs on Jan. 4, when Jupiter is at opposition. This means that as seen from the Earth, Jupiter is opposite the sun, meaning that Jupiter, Earth, Venus, and the sun are all on a line. At opposition Jupiter is at its largest apparent size and brightest appearance.
Therefore, both Venus and Jupiter, the brightest planets, will be at their biggest and brightest during January.
In the first two weeks of January you can even compare them. Jupiter will rise in the east-northeast shortly before Venus sets in the west-southwest.
So in January, watch Venus change from the evening star into the morning star, and see if you can find both Venus and Jupiter in the post-sunset sky during the first week of the month.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.