orbits

The orbits of planets and asteroids in our solar system. The orbits of Mercury and Venus are shown in gray, closest to the sun (center).

Last month I told you how to find Venus and Mercury in the evening sky just after sunset. All we needed were a few clear nights to see them, and we did get those. I hope you had a chance to see both planets.

If you look for these two planets now, you will see Venus, but Mercury is gone. Where did it go?

The orbits of both Mercury and Venus are closer to the sun than the orbit of the Earth. We call them inferior planets. The closer a planet’s orbit is to the sun, the shorter its orbital period.

Mercury is closest to the sun and has an orbital period of 88 Earth days. The orbit of Venus is between Mercury and Earth and has a period of 225 Earth days. And of course, the Earth’s orbit of the sun takes one year.

As seen from Earth, Mercury — with the closest distance and shortest orbital period — stays the closest to the sun, while Venus — with a greater distance and longer period — appears to travel farther from the sun.

At the beginning of March, both planets were traveling away from the sun, but on March 15, Mercury reached its maximum eastern distance and started its journey back toward the sun. By now it is too close to the sun for us to safely see it. Mercury passes between the Earth and the sun today, April 1.

In astronomy we have names for some of these special positions of the inferior planets. When an inferior planet is between the Earth and the sun, moving from the evening sky into the morning sky, it is at inferior conjunction. This is the case for Mercury on April 1. A superior conjunction occurs when the Earth, sun and one of the inferior planets are in line, with the inferior planet on the far side of the sun.

The inferior planets therefore go through cycles: A planet is seen in the west after sunset for several weeks or months, then in the east before sunrise for several weeks or months, and then in the west after sunset again.

The angle between the sun and a planet as viewed from Earth is called the planet’s elongation. When the planet’s position in the sky after sunset is as far east of the sun as possible, it is at greatest eastern elongation. The planet will appear at its maximum distance above the western horizon and is often called an “evening star.”

At greatest western elongation, Mercury or Venus is as far west of the sun as it can possibly be. The planet will rise before the sun and is often called the “morning star.” The planet will be at its maximum height above the horizon at dawn in the eastern sky.

The other three naked-eye planets are Mars, Jupiter and Saturn. These planets are called the superior planets because their orbits are beyond the Earth’s orbit. The inferior planets are never visible at midnight because they never venture that far from the sun, but the superior planets can be visible in the sky at any time.

When a superior planet is in a part of the sky behind the sun, we say it is in conjunction. Because you should never look directly ay the sun, this is not a safe time to try to look at the planet. A planet in conjunction is never visible at night and can only be found in the daytime sky.

Because the orbits of the superior planets are beyond the orbit of the Earth, they are never at inferior or superior conjunction. When one of the superior planets is in the part of the sky directly opposite the sun, we say it is in opposition. The planet is completely illuminated by the sun, making this the location where it appears to be brightest.

This is also the location where it is closest to the Earth. A planet at opposition will be highest above the horizon at midnight local standard time. Opposition is the best time to view these superior planets because you are looking through the least amount of the Earth’s atmosphere.

Jupiter will reach opposition during the second week in May, on May 8. Jupiter moves slowly in its orbit; it takes almost 12 years to complete one orbit around the sun. A planet this far from the sun will not change much during April or May; the increase in brightness during opposition will be almost imperceptible.

But remember that Jupiter should be due south at your location at midnight local standard time. On a clear night during the week of May 8, see if you can find the planet and check for its location at midnight.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at  Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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