May 20, 2022: The moon approaches the morning planets that are scattered along the eastern sky. Can the planets ever appear along a line that extends from the sun?
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 5:26 a.m. CDT; Sunset, 8:09 p.m. CDT. Check local sources for sunrise and sunset times for your location.
As sunrise approaches, the gibbous moon, over 20° above the southern horizon, is approaching the morning planets. Saturn is less than 30° to the upper left of the lunar orb. This morning the moon is 76% illuminated.
Observed during early twilight or before, the moon still illuminates the ground and casts shadows.
The four morning planets continue to stretch across the sky, from brilliant Venus to Saturn, spanning 55.3°. Notice that the planets and the moon are in a line. This is the plane of the solar system, the ecliptic.
Venus is “that bright star” low in the eastern sky during morning twilight. Bright Jupiter is 18.6° to the upper right of Venus and Mars is 5.2° to the upper right of the Jovian Giant. The step to Saturn from Mars is 31.5°.
Mars is marching toward a conjunction with Jupiter on May 29 before sunrise.
While the planets are lined up in the sky, they are not in a line as seen north of the solar system, a view never seen by humans.
The planets are on the same side of the sun, but not in a line. From our view at home, we see them lined-up along the ecliptic. From the sun’s perspective, they are millions of miles apart and scattered along an angle that measures 32.4° from Venus to Jupiter.
From the center of the solar system, Jupiter is farther eastward in its orbit than Venus, the most westward planet from this view. In the skies of Earth, Venus is farthest eastward and Saturn is westward.
Mercury is still on the evening side of the sun, hiding in the central star’s bright light. Next month, it moves into the morning for a five-planet spectacular.
With the planets over 32° apart viewed from the center of the solar system, is there a time when the classic nine planets are in a straight line.
This leads to a question as to whether the planets are ever on a line extending from the sun. Astronomy Jean Meeus tackles this question in his book Mathematical Astronomy Morsels.
The challenge is that the planets do not move at the same speeds around the sun. Mercury revolves around the sun quickly and Pluto revolves once across several human lifetimes. Additionally, the speed a planet moves through its orbit is not constant. In their slightly elliptical orbital paths, the planets move faster at their perihelion points than when they are farther away.
The problem is fairly easy to determine for two planets, but when individual planets are added – including the other seven, the question is more difficult to answer.
The author states that the simplest answer is “never.” If the planets are put into circular orbits with constant speeds, the answer is 3 times 10 to the 17th years, the number three with 17 zeroes after it. The age of the solar system has nine zeroes. His simple answer of “never” seems appropriate for this question.
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