Way

We discussed the points in the sky called the Apex and Antapex of the Earth’s Way.  They are the points ahead and behind, at a given moment, in our travel around the Sun.  We had some fun with less intimidating names for them.  At least, Shannon Templeton’s EDOT or Earth’s Direction of Travel is clear enough for the point ahead; for the rear-view point, I don’t think any of the suggestions so far will quite succeed in superseding that resoundingly popular term, that phrase on everyone’s lips, the Antapex of the Earth’s Way.

There is another possibility, as you’ll see at the end of this: the Earth’s Quit.

But I like to include the word “Way.”  Why?  It was the handy syllable I found, back when I was trying to work out how to plot the motions of the stars.  One often needs a group of letters, as short as possible, for tracing a concept through a mass of programming (maybe to find where it is going wrong).

Just as the Earth has a Way, with a forward and a backward direction, so does the Sun and so do the other stars.

Here is how the stars are moving.

This picture has to be rather large; I hope you can zoom in on it; I’m also putting it into a bonus “Deeper Space” page in “Astronomical Calendar 2017” (see the links above), where it is in a form that you can enlarge.

The stars shown are a very few of those around us.  They are those that are in the “top twenty” of apparent magnitude (brightness as seen from Earth) and are within about 100 light years from us.  That is why they crowd toward the middle: the Sun and its neighbors such as Alpha Centauri are relatively small stars, appearing so bright only because of their nearness.  Outside the sphere of space shown are giant stars that are bright for us despite their distance, such as Canopus, Betelgeuse, Rigel, and Deneb.

But it’s clear which way we’re all going.

The grid lines serve to show the scale: they are 50 light years apart.  They are drawn on the equatorial plane (in which Earth rotates); I could have added the ecliptic plane (in which the Sun’s little family revolves) but that might be confusing.

The labels at the ends of some grid lines (they’re painted on the inside of the imaginary sphere) are the cardinal directions, 0 and 6 and 12 and 18 hours of right ascension, so they tell you which way everything is oriented.  0h, to the right, is the reference direction, the vernal equinox (toward which we face the Sun on March 20).

The hoop made of dots represents the plane of our Milky Way galaxy.

The arrows from the stars represents their motions to where they will be 100,000 years into the future, relative to their present position in the galaxy.

They are in orbits around the center of the galaxy, to which the red arrow points.  The galaxy as a whole is rotating that way – clockwise as seen from what we call north (in contrast with the solar system’s counterclockwise-as-seen-from-north rotation).

The stars’ journeys must be slightly curving, but the difference from straightness would be far too small to notice, because 100,000 years is a tiny fraction of their orbital periods of somewhere around 225,000,000 years (the “galactic year”).

Actually some different kinds of motion are involved.

One is the collective motion of this part of the galaxy, in the galactic plane and at 90° to the galactic center.  If we were showing this, the arrows would all be parallel.  This average motion is indicated by the orange arrow at upper right.

But each star has its own slightly different motion.  It may be, in this part of its orbit, drifting slightly inward or outward, northward or southward, and slower or faster than the Sun is.  Astronomers can observe and measure this.  Thus for each star there is a “proper motion in right ascension” and “proper motion in declination”; in other words, change of position per year on our map of the sky; measured in seconds of right ascension or declination, because it is very small for nearby stars and even smaller for distant ones.  And there is its radial velocity, that is, movement toward or away from us, in kilometers per second of time, found from the shifting of lines in its spectrum.  Putting these together, we can find the star’s spatial change of position, relative to us, per year.  I’ve used the figures given in the Hipparcos catalogue.

Then there is the Sun’s own motion relative to the local swarm.  It is represented by the yellow arrow.

So we can talk of two kinds of apex (I leave it to you whether to say “apices” or “apexes”), two kinds of ahead-point.  One is the Apex of the Stars’ Way, which has to be in the  galactic equator and 90° from the galactic center.  The orange arrow points to it.  The other is the Apex of the Sun’s Way, to which the yellow arrow points.

Where are these on the map of the sky?  The first (galactic latitude 0, longitude 90) corresponds to right ascension 21h 12m, declination +48° – in Cygnus, not far from Deneb.  The Apex of the Sun’s own Way is at 18h 06m +30° – in Hercules but not far from another brilliant star, Vega in Lyra.

So we are aiming, in the present part of our rather sinuous galactic orbit, toward where Vega is now.  But in the longer term, we and our neighbors are destined to be about where Deneb is now – more than three million years into the future.  For Deneb is thought (with a good measure of unsurprising uncertainty) to be around 3,000 light years away, so that, to be the twentieth brightest star in our sky, it must be one of the most luminous supergiants in the galaxy.  Amazing to have such a beacon to mark our destiny ahead.

There is more about the stars’ Way in the Astronomical Companion, near the beginning of the section called “Outrush.”

Oh, and the point about opposite to Vega on the celestial sphere, in the northeast corner of the constellation Columba the “Dove” and not far southwest of Sirius, is the Antapex of the Sun’ Way.  But it also has been called by a phrase that sounds so traditional as to be almost Chaucerian: the Sun’s Quit.