January is the month that the Earth moves the fastest around the sun. Or looked at from our perspective, the sun moves faster through the sky in January. Every day the sun moves a little less than 1 degree of angle against the background stars, assuming you could see the stars in the daytime. But when the Earth is closer to the sun, this rate is a little faster. On Jan. 2, the Earth was at perihelion, or its closest approach to the sun, so this day would show the most motion.
Last month, I briefly discussed planetary orbits as being elliptical, which is what Johannes Kepler’s first law states and Isaac Newton’s laws of motion can calculate. But perhaps a more significant contribution from Kepler was his second law about speeds in orbit. For planetary conjunctions to take place, planets not only need the right path through the sky, but they need to arrive at the right time. Although this isn’t how he stated it, Kepler’s second law says planets move faster when they are closer to the sun and slower when they are farther away. In Newtonian mechanics, this just means that angular momentum is conserved.
The sun moving at about a degree per day completes its trip through the sky in a year. It is not a coincidence that there are 365 days in a year and 360 degrees in a circle. Ancient calendars used 360 days, and because 360 can be divided much more easily than 365, we should all be glad that 360 is still used for angular measurements.
This monthThe new moon is on Jan. 12, and this should be the best time to enjoy the winter sky with all the bright stars and constellations surrounding Orion. The belt stars are rising in the east after sunset and will be at their highest point to the south a little after 10 p.m. At the same time, the Milky Way will be just to the left of Orion and pass through the zenith point.
A year ago, the big news was that Betelgeuse, the bright red star of Orion’s shoulder, was much dimmer than expected. It had long been known to be a variable star, but in January 2020 it was a full magnitude, or about 2.5 times, dimmer than its historical range. It got brighter again by late summer, which suggested it was back to its regular variability, but you should check to see if you think it has returned to its traditional brightness.
Even though this next conjunction comes in February, it will happen before my next column comes out, so this is the only time to mention it.
Between Feb. 5 and Feb. 11, Venus will pass Saturn and then Jupiter in the morning sky just before dawn, so there will be another planetary conjunction, but this time a triple. The almost-new moon and the planet Mercury will also be in the same part of the sky, so you might be able to see even more solar system objects joining the party. My sky chart shows that Pluto is also in the area, but don’t expect to see something that shows up only as a dot through the Hubble telescope unless you also have a large scope and know exactly where to point it.
On the morning of Feb. 11, Venus will be less than half of a degree from Jupiter, so you could easily cover both with your pinky finger held at arm’s length. Jupiter will be about the same brightness as during the conjunction last December, but for this conjunction, Venus will be the brighter one. It will be almost six times brighter than Jupiter, and about 65 times brighter than Saturn was in December.
There will not be much time to view these planets on Feb. 11, since Venus doesn’t rise until 6:39 a.m., and the sun rises at 7:04 a.m.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at [email protected]