The Quadrantid meteor stream hits us in the night of Jan. 3/4. Here is the picture from Astronomical Calendar 2016 page 6, but more expansively and with an addition.
Shannon Templeton, in Durham, North Carolina, wrote to me that she has “a love of astronomical motions [and] a passion for writing astronomy programs” but was puzzled over how to find the position in the sky of what she calls the “Earth’s Direction of Travel,” or EDOT: “the imaginary, ever changing point we are racing towards.” She had expected various trigonometric complications arising from our position on the spherical and spinning Earth, and the ellipticity of Earth’s orbit.
I think (someone may correct me) that there is no complication. The direction is simply that which is tangent to Earth’s orbit at the moment: in other words, it is in the ecliptic plane and at ecliptic longitude 90 degrees west of the Sun. For instance at 2016.0 the Sun is at longitude 280 degrees, so the EDOT is at longitude 190, latitude 0. No matter where you stand on Earth, you’re being carried toward this point.
Shannon replied, in effect, “Of course. It’s simpler than I thought.”
I hasten to add that Shannon is not just an enthusiastic but an advanced student of astronomy. She knows more than I do, and was once engaged in the study of binary-star motions and their apparent divergence from relativity.
It struck me that her “EDOT” is worth adding to sky illustrations (like the anti-Sun or “Earth’s shadow” point which I had already added and which is at a right angle to it), especially because of her further remark: “One reason the EDOT intrigues me is so I can visualize where the radiant of a meteor shower is compared to the EDOT, thus showing the angle of the comet debris orbit. This is of zero importance to anyone on the planet, but these are the kinds of things that keep me awake at night until I figure them out.”
Yes, it’s far from immediately obvious how the direction from which the Quadrantids, or other meteors, come toward us – in other words their radiant, which gives them their name – results from their curving orbit and its intersection with our curving orbit. In trying to visualize it better, this was the first diagram I plotted:
– a sort of close-up of another Astronomical Calendar illustration, the one showing the orbits in space.
The dotted line represents the orbit of the meteors, though strictly – since the meteor stream in space is millions of miles wide – it is the path only of those few meteors that happen to arrive from the zenith.
And the Earth’s motion is shown by the thick arrow, a sort of rail along which Earth is riding in its journey around the Sun. (It’s like the flight-of-Earth arrow in the Astronomical Calendar pictures for solar eclipses, but longer.)
It is now fairly easy to see the relation between these two paths in space. But not easy to measure, since it is an angle in a solid picture.
But look back at the first picture, the sky scene. The distance between “the radiant of the Qudarantids” and “Earth’s direction of travel” is the angle between them.
I realized that I could make my plotting program spit out the position it had calculated for both of these points, and then calculate the angle between them. The radiant is 63 degrees from the EDOT. (And at a position angle of 71 degrees from it, as measured from the zenith. It could be given in other ways, as from the north celestial or north ecliptic poles.)
All that may not be particularly vivid to anyone other than me and Shannon. What surely twangs a note of excitement is that target-like symbol. We are all hurtling, at 67,000 miles an hour, toward that point.
Notice that the EDOT just about coincides with the position of the Moon on Jan. 2 when it was at Last Quarter. Naturally: for when the Moon is at that position, it is passing around in front of us, it’s “left” (west) side illuminated by the Sun.
As the remaining hours of the night go on, the point will move very slightly eastward down the ecliptic; and the horizon will move much faster downward; until, at the moment when the Sun is exposed, we will be on the “bow” of our Earth ship, our heads pointing up in the direction in which we are going.
– I had to hit “publish” to this post without giving it the usual preview, and go on a visit. Returning three hours later, I notice that the concave horizon in the sky scene makes it less easy for you to sense that we are on a spherical planet rolling forward as it hurtles toward that point in Virgo, that it will roll forward to reveal the Sun. Easy: I just change a number (the altitude of the viewpoint) from +25 to -5.
It’s an arbitrary number. The horizon is a great circle: it’s concave if you think of it as surrounding the sky, it’s a straight line if you look at it, it’s convex if you think of it as part of the surface of our rolling and hurtling globe.