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Zodiacal light 2020
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08-03-2020 8:58am#1
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For this discussion, no. It only matters that the zodiac is a continuous belt in the plane of the ecliptic. Precession causes the belt to rotate with respect to Earth's seasons. But the zodiacal light is produced by dust in the plane of the ecliptic which is there no matter which direction in the plane you look.Capt'n Midnight wrote: »Doesn't precession mean the Zodiac is in the wrong place ?
There is a reason why the light is seen most easily after sunset in springtime and before dawn in autumn. It doesn't mean the belt isn't continuous. On this -- as on so much else -- the OP is wrong.
But it's a tad difficult to visualise. First, let's recap. The ecliptic is essentially the plane in which the planets orbit the sun. When we look out at it from earth it's a line in the sky along which the sun and planets seem to move, and on which they will always be found. The zodiacal constellations are also on that line, but that's because they were specifically chosen for that purpose.
But although the earth orbits in the ecliptic, its equator is not in that plane as its axis is tilted. That means the ecliptic line is not in a fixed position in the sky as the earth rotates each day, but it swoops and dives. One picture which can be misleading if we're not careful is this one:
This shows the daily path of the sun for different dates for an observer in the northern hemisphere. Look at the sun's path for the equinoxes. It rises directly east, tracks across the sky to a highest point in the south (determined by the observer's latitude) and then sets directly west. It is tempting to think that this track across the sky is the ecliptic. But this is most definitely not the case. If it were, the zodiacal light would show no preference for sunrise or sunset, spring or autumn.
Even though the sun is on the ecliptic at all times, the ecliptic itself is moving. Here is the ecliptic line just before sunrise on the March 2020 equinox as seen from Dublin. Indeed, you can see several planets arrayed along it.
Now here is the ecliptic just after sunset on the same day:
The ecliptic is in a completely different place at sunrise and sunset, although it transitions from one orientation to the other smoothly as the day progresses, so that the sun's actual track is the neat symmetrical one you saw in the first picture. (You can set up Stellarium to see all this in live motion).
Before we put all this together, let's look at one more view. Here's the earth's position in its orbit throughout the year:
At the March equinox the axial tilt is neither toward nor away from the sun, but edge on. Imagine you're an observer on the west coast of North America which is just swinging into view on the earth's top left "shoulder". Roughly speaking, you are standing on top of the world so the ecliptic plane is oriented along your horizon. Twelve hours later the earth's rotation has carried you around to the side of the world where the ecliptic is perpendicular to your horizon in the western sky.
To sum up: at the March equinox the ecliptic line is oriented at a shallow angle to a northern observer's horizon at sunrise. It is oriented at a steep, almost vertical angle to the horizon at sunset:
Now let's think about where the zodiacal light is coming from. It is sunlight reflected off dust which is moving primarily in the ecliptic plane along with the planets. Indeed, it is dust left over from the formation of the planets and from countless grinding collisions since. In order for the reflected sunlight to reach us here on earth, we need it to be forward scattered from dust between us and the sun at a shallow angle:
So we need the dust to be at a small angular distance from the sun so that it scatters light toward us. But we need it to be at a large angular distance from the sun so that we can put the sun below our horizon and see the scattered light in a dark sky.
And there's our explanation -- when the ecliptic is at a shallow angle to the horizon the sun will never be far below, so the dust gets lost in the glare. But when the ecliptic is at a steep angle (such as after sunset at the March equinox) the sun can get far enough below the horizon to give us a dark sky view of the scattered zodiacal light. And, of course everything is reversed in the autumn.
For a bonus, we've also explained a whole other phenomenon -- the Gegenschein -- which comes to us from the opposite direction. This is a type of zodiacal light reflecting back to us at acute angles from a point opposite the sun. Think of how the full moon works when the moon is opposite the sun in the sky. The gegenschein is bazillions of tiny dust particles, each lit up like a full moon.0 -
But although the earth orbits in the ecliptic, its equator is not in that plane as its axis is tilted. That means the ecliptic line is not in a fixed position in the sky as the earth rotates each day, but it swoops and dives. One picture which can be misleading if we're not careful is this one:
This shows the daily path of the sun for different dates for an observer in the northern hemisphere. Look at the sun's path for the equinoxes. It rises directly east, tracks across the sky to a highest point in the south (determined by the observer's latitude) and then sets directly west. It is tempting to think that this track across the sky is the ecliptic. But this is most definitely not the case. If it were, the zodiacal light would show no preference for sunrise or sunset, spring or autumn.
Even with very little traffic on the forum during this health crisis, it is becoming increasingly difficult to respond without being bounced out of the forum which often gives you an open goal for a period. I shrug as that leaves me with a blog instead of a genuine discussion so therefore it isn't really a forum with such suffocating strictures present.
Solstices and Equinoxes are orbital terms as they refer the relationship of the North and South poles both to the planet's circle of illumination and to the central Sun. In this case the ecliptic plane becomes the benchmark for organising perceptive narratives. I can supply 3 out of the 4 relevant positions of the North/South poles with special attention to the Equinox point -
https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory#/media/File:EpicEarth-Globespin-tilt-23.4.gif
The ecliptic Equator representing the orbital plane would run across the maximum circumference of the Earth, so that at the September Equinox the orientation runs from Northwest to Southeast relative to the orbital plane and Southwest to Northeast on the March Equinox just gone by. Likewise the Solstices (where the dotted line represents both the ecliptic plane with a line running across the Earth maximum circumference) -
https://en.wikipedia.org/wiki/Axial_tilt#/media/File:Axial_tilt_vs_tropical_and_polar_circles.svg
https://www.youtube.com/watch?v=-OgLCH7jYp8&t=63s
The North and South poles turn parallel to that dotted line as a function of the orbital motion of the Earth so, seeing you are talented with graphics, you can represent the differing orientations of the North South poles as this exist on the circle of illumination at opposing Equinoxes. The trick is to keep the fully illuminated face of the Earth in view as they would be seen from the orbital plane to render the orbital Solstice/Equinox points accurately.
The celestial sphere graphics make no distinction between March/September Equinoxes hence their less than global view of the matter.0 -
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