Meet The Ecliptic, One Of Scariest Things In Astronomy

About 20 minutes after sunset watch for the pair of Venus and Mercury (below Venus) low in the southwestern sky. Maps created with Stellarium

Planet watchers with a clear view to the west-southwest can find Venus and Mercury tonight through next week. Venus is much brighter and easier to see and located about five degrees (three fingers held together horizontally at arm’s length) in the southwest about 20 minutes after sunset. It’s well to the left of the bright glow along the horizon that marks the location of the sun. Mercury is directly below Venus and nearly lost in the twilight. You might be able to see it with your naked eye, but bring binoculars just in case.

Mercury reaches greatest angular distance from the sun Monday. As seen from Earth, it's well off to the left or east of the sun at dusk. Telescopes will show a "half-moon" phase. Credit: ESO

Mercury reaches greatest elongation from the sun this coming Monday. Normally, when it’s so widely separated from the sun, we can see it with relative ease at dusk. But because the angle of the ecliptic – the path the planets, sun and moon follow –  is so shallow in the western sky in November, Mercury nearly scrapes the horizon. Venus does only a little better.

Mercury and Venus travel along the ecliptic, which is tipped at a very low angle in the western sky in November. For southern hemisphere observers, the ecliptic is shifted up and to the right, making a steeper angle with respect to the horizon.

Southern hemisphere observers see Venus and Mercury very differently this month. Just as the season are reversed, at southern latitudes, the ecliptic is tipped up at a steep angle and Mercury is easily visible at dusk. Mercury is best visible during evening hours for northern hemisphere observers during the spring months, when the ecliptic stands high in the west.

The ecliptic is the big circle the sun moves along in the course of a year. Also shown are the celestial equator - an extension of Earth's equator into space - and the north and south celestial poles, extensions of our axis into space. Credit: Joshua Cesa

Notice in the diagram above how the sun lies exactly on the ecliptic. That’s because this imaginary circle is defined as the plane of the Earth’s orbit around the sun. Since the sun’s movement is really just a reflection of Earth’s orbital motion, the ecliptic can also be viewed as the path the sun takes through the sky in the course of a year. And that path is a big circle inclined to our planet’s equator by 23.5 degrees.

Why 23.5? That’s the tip of Earth’s axis. That tip is projected on the sky just like our orbit. It’s what causes the sun to appear high in the sky in summer and low in winter.

Each day the sun moves about one degree or two full moon diameters eastward in the sky. Of course the sun’s not really moving. The Earth, speeding along its orbit at 18 1/2 miles per second, travels 1.6 million miles a day. From our ever-changing perspective, the sun appears to slowly slide east along the ecliptic.

Now you might be wondering why the planets and the moon also drive the same highway as our planet does. Turns out their orbits are tipped only a little bit with respect to Earth’s. In other words, all eight planets essentially lie in the same flat plane. As each orbits the sun, they follow the same familiar ecliptic path and pass through the same dozen zodiac constellations.

Mercury’s orbit is tipped the most with respect to the ecliptic at about 7 degrees. That’s why it’s farther than Venus (3.4 degrees) as shown in the ecliptic illustration above. Uranus varies least with an inclination of just 0.8 degrees.

Because the moon's orbit is tilted 5.2 degrees (exaggerated for effect here), it only lines up to create eclipses when it's passes through the ecliptic at new and full moon. Credit: C.R. Nave, Georgia State University

The word ‘ecliptic’ derives from eclipse. When the moon, which has a slightly tipped orbit, intersects the ecliptic at full moon phase, the sun, Earth and moon are exactly lined up and a lunar eclipse occurs. When it intersects at new moon, the sun, moon and Earth (in that order) are exactly lined up and a solar eclipse occurs.

It’s fascinating how the interplay of orbits creates so many fun events to watch for in the sky. Understanding the ecliptic isn’t always easy – I’ve seen a few pairs of eyes glaze over –  but once you do, you’ll be on the path to astronomical enlightenment.

Speaking of eclipses, the next one’s coming up very soon! A total lunar eclipse will occur in the early morning hours of December 10 for the western half of the U.S., Canada, Australia and central and eastern Asia. I’ll provide more details and viewing tips as we get closer to that date.

3 Responses

  1. thomas s

    hi Bob. and it’s quite fortunate that the Earth’s tilt is 23.5 degrees. makes for a more congenial climate. if it were tipped 90 degrees, for example (like Uranus or is it Neptune), I think that there would be perpetual light and heat on one end and perpetual dark and cold on the other. and so on. hope I have this right. anyway if the tilt were much different than it is, things would be a lot different than they are (ceteris paribus of course)

    1. Dave Mowers

      It might have been that way in the past as an asteroid the size of which is believed to have created the Gulf of Mexico could have not only killed off the dinosaurs but caused the wobble and tilt as well. This could explain the ice ages.

      1. astrobob

        While that was a significant hit, the dinosaur-killing asteroid was not the reason for Earth’s precession wobble or the tilt of the planet’s axis. The first is caused by the tug of the sun and moon on Earth’s equatorial bulge. The tilted axis goes back to the early Earth and was most likely caused by a small planet hitting our own. It may have been the same event that ultimately created the moon.

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