Mercury-Jupiter Conjunction of Friday March 5, 202

Eric Saltmarsh

If you went outside just after dawn on Friday, you were treated to a close conjunction of the planets Mercury and Jupiter, in the constellation Capricorn. The planets rose together in the ESE about 5:15am MST.

On that morning, Mercury and Jupiter were separated by only 0.6 degrees. Jupiter, at magnitude -2.0, is the brighter planet, with dimmer Mercury at magnitude 0.6. Saturn, magnitude 0.7, is also in the vicinity, to the pair’s upper right.

If you look for these planets on more than one morning, notice how quickly Mercury and Jupiter appear to converge before conjunction, then separate afterward. Mercury is the real speedster here since, by virtue of its proximity to the Sun, it has a much higher orbital speed than distant, sluggish Jupiter.

These relationships are explained by Johannes Kepler’s three laws of planetary motion, published early in the 17thcentury:

  1. The orbit of a planet is an ellipse, with the Sun at one of the two foci.
  2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of a planet’s orbital period is proportional to the cube of the length of the semi-major axis of its orbit.

Cutting to the chase, Kepler’s laws confirm that planets orbiting closer to the Sun travel faster than planets farther from the Sun.

In the case of Mercury and Jupiter, Mercury, the innermost planet, travels around the Sun at 107,000 miles per hour versus Jupiter’s 29,000 miles per hour. Mercury’s orbit is much smaller than Jupiter’s, as well. Since a year is the time it takes a planet to complete one revolution around the Sun, a year on Mercury is equivalent to 88 Earth-days. A year on Jupiter is equivalent to 11.9 Earth-years.

Mercury & Jupiter, Tuesday March 2
Mercury & Jupiter, Tuesday March 2
Mercury & Jupiter, Wednesday March 3
Mercury & Jupiter, Wednesday March 3

Both phots were taken at the same time. Jupiter is higher on the 3rd because it rises earlier each day. Mercury looks like it hasn’t moved in the last 24 hours, but in actuality, it has reduced its apparent distance from Jupiter by one-third.