04 January 2011

Beginning anew (orbitally speaking)

Where are we as we begin the new year?

Even though it may not feel like it in the northern hemisphere, yesterday (January 3) the Earth passed through perihelion, the point in our elliptical orbit that is closest to the Sun (3.4% closer than at aphelion in July). Demonstrating again that on Earth the seasons have more to do with the tilt of our axis than with our distance from the Sun.

For those of us living near latitude 40° North (e.g., Quincy, CA and Philadelphia, PA), tomorrow (January 5) is the latest sunrise of the year. Yes, I know that the shortest daylight in the northern hemisphere was back in December on the solstice; nevertheless, the Sun's actual position in the sky is not the same as its average position which is what clocks measure. The difference from day to day is due to the fact that the Earth goes faster along its orbit when it is closer to the Sun and slower when it is farther away. Combining this effect with the tilt of the Earth's axis gives the equation of time which can be plotted as the analemma (the figure-8 thing on your globe).

Coincidentally and completely unrelated to the above facts or each other, both Venus and Mercury happen to be at their greatest elongation this week, Venus on Jan. 8 and Mercury on Jan. 9. "Greatest elongation" means that they appear in the sky as far from the Sun as is possible. Remember that both Mercury and Venus orbit closer to the Sun than Earth, so if you drew a triangle with the Earth, the Sun, and Venus (or Mercury) as vertices, then at greatest elongation the angle at Venus (or Mercury) will be a right angle, 90°. This week the angle between the Sun and Venus seen from Earth will be 47°, and 23° for Mercury.

If you look at Venus this week with good binoculars or a telescope, you would see it is half. Like the moon, Venus has phases. It is these phases of Venus that Galileo observed about four centuries ago, convincing him and others that the Earth and the planets orbited the Sun and that the Earth was not the center of the universe.

Have a happy new year!