East of the Sun. West of the Moon

© 2002 by Robert C. Moler

East of the Sun, West of the Moon
By Bob Moler

Not too long ago I got this question emailed to me: What's east, what's west?

When a planet is seen in the evening sky, they say the planet is east
of the sun and when you see a planet in the morning sky, they say the
planet is west of the sun. My question is what is the dividing line
that makes a planet east or west of the sun? I guess north or south
would be referenced to the sun's equator but how could a planet be
east or west of the sun? 

noon2.gif (8057 bytes)I dashed off a rather quick reply, but I didn't explain it to his or, for that matter, my satisfaction. He suggested using diagrams. So here goes.

More than once I've noticed something odd about the statement that when a planet, like Venus is at greatest eastern elongation from the sun, as it was last month, is visible in the west. I hoped it wouldn't confuse anyone for too long.

As far as directions go, north and south don't go on forever there's the north and south poles. Go past the north pole and you're heading south. It reminds me of a riddle: There's a square house with four south facing walls. A bear can be seen outside one of the windows. What color is the bear? It's white, of course. The house has to be at the north pole, so the bear is a polar bear.

Anyway, the east and west directions go on forever, or at least round and round. To be practical, since there's 360 degrees in the circle or celestial sphere, an object is east or west of another when their separation in that direction is less than 180 degrees, half or 360.

In the chart above we are looking at the sun and planets at local noon on September 22nd 2002. This is the same chart as our monthly star chart. It's like looking at the sky with a fish-eye lens so that the horizon is a ring. I've also removed the stars. With the sun due south of us, nearly on our meridian, what east of the sun is east of us too, The planets Mercury and Venus are east of both the sun and us, while the planets Mars, Jupiter and Saturn are west of the sun and us.

sunset.gif (7563 bytes)Since the planets lie near the ecliptic, the apparent path of the sun, they are rarely due east of west of the sun, but that doesn't really matter for our discussion, since we are talking about the general east and west directions. When the sun sets the planets Saturn, Jupiter and Mars set before the sun leaving only Venus and Mercury as evening planets. This is seen in the diagram on the left at sunset. Note that in the evening in late summer and early autumn that the ecliptic rides low in the south. The planets in the western part of the sky are seen close to the horizon, even if the are at some distance from the sun.

Venus, on the 22nd of September 2002, the date of these charts will be 41 degrees east of the sun. If it was due east of the sun, it would set nearly 2 and three quarters after the sun. However, since it is also south of the sun will set only 61 minutes after the sun.



sunrise.gif (7119 bytes)The morning sky, as seen in the chart on the right, is more conducive for seeing planets close to the sun now, because the ecliptic rides high in the south. Saturn in our chart is about where the sun is at noon on the first day of summer, about 67 1/2 degrees above the southern horizon. Mars and Jupiter, which are west of the sun are in the eastern part of the sky. Saturn is a bit more than 90 degrees from the sun and will move from the eastern to the western sky around sunrise.

Saturn here shows that a planet west of the sun can appear in the west also if it is far enough west, that is greater than 90 degrees west of the sun. Of course in the evening a planet greater than 90 degrees east of the sun will appear in the eastern sky at sunset. Of course over the evening the planet will move, with the rest of the sky to the west.

I hope that explains why though west is west and east is east they may indeed meet.

Questions? Send Email to me at bob@bjmoler.org

Updated: 09/02/02