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 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.
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.
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.
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.