Inner planets at perihelion

¡SkyCaramba! Weekly astronomy blog for the week ending April 18, 2015

Mercury, Mars, and the Moon on April 19, 2015

Venus and Mercury both happen to be very close to the sun this coming weekend. Both inner planets will be at perihelion the weekend of April 18-19. Venus is first. Then Mercury.

For an object orbiting the sun, perihelion is the point when it’s closest to the sun. All the planets, asteroids, and comets orbiting the sun go through perihelion once per orbit. Also once per orbit, they go through aphelion which is a point that’s farthest from the sun.

Since our calendar is tied to our planet’s orbit around the sun, Earth goes through perihelion on or about January 3 every year and aphelion on or about July 3. The other planets’ perihelions and aphelions could occur on any date though. Jupiter last went through perihelion on March 18, 2011 and will do so again on January 20, 2023. Mercury goes through perihelion four times in 2015 and Venus does twice.

By coincidence Venus goes through perihelion on April 18 and Mercury the next day. Venus will be about 107,000,000 kilometers from the sun. Mercury will be 46,000,000 kilometers away. At their farthest from the sun, Venus is 109,000,000 away and Mercury about 70,000,000 away.

Comparing those orbital extremes, you readily see that Venus stays about the same distance from the sun throughout its 225-day orbit. Its orbit is nearly circular. Meanwhile, Mercury puts tens of millions of miles more between itself and the sun from perihelion to aphelion. That orbit is a much more distinct ellipse.

If an object can orbit in a perfect circle around the sun, it will have no perihelion or aphelion. Perhaps a spacecraft will be in such an orbit someday. For now, there are many satellites in circular orbits around Earth. But for objects orbiting our planet, we use the terms perigee and apogee in place of perihelion and aphelion, respectively.

Many comets go through perihelion but not aphelion. The sun’s gravity draws them close. But they are moving too fast to stay in orbit. It’s possible that many of them were in very distant orbits before being nudged toward the sun and then escaping to interstellar journeys.

You can see Mercury and Venus in the evenings this weekend. Mercury will be hard. You’ll need a clear horizon with no distant clouds spoiling the view right after sunset. Venus is much easier to spot higher in the sky but in the same general direction above the place where the sun went down.

Venus, the Hyades, and the Pleiades on April 18, 2015