Astronomy/Movement of the Sun through space
QUESTION: Dear Philip
I wonder if you could please tell me the angle of inclination between
1) The Ecliptic Plane and
2) The plane of the Earth's orbit around the galaxy.
Also, do you know when if and when the latter was parallel with the Earth's Equatorial plane during the Earth's cycle of nutation?
Finally, which of the Earth's poles is pointing in the direction of the Sun's motion more than the other?
Thank you so much for sharing your expertise.
The first question is problematic since it presumes a relatively set plane of Earth's orbit (and hence Earth-Sun radius vector)i.e. in relation to the ecliptic plane, say analogous to the Earth's polar axis in relation to the ecliptic plane. But as seen from Earth, the Sun is observed to be south of the galactic equator (or galactic plane# for half the year, and north of the galactic equator for the other half of the year.
Meanwhile, on or near the solstices #June 21, Dec. 20# the Sun crosses the galactic equator. At the former, the Sun appears to cross the galactic equator going from south to north. Six months later, the Sun appears to cross the galactic equator going from north to south. Hence, the ecliptic plane #plane of Earth's orbit# can actually briefly coincide with the galactic plane defined as being at the galactic equator.
To estimate the Sun's maximal north-south divergences from the galactic plane, you can use Norton's Star Atlas. This source features a Galactic Chart which shows the celestial coordinates of the northern sector of the sky with the Milky Way's galactic coordinates superposed upon them. From this one can see that the North Star #Polaris# is roughly at galactic latitude 27 degrees, i.e. 27 degrees "north" of the galactic equator #though technically, the cardinal directions don't have the same meaning they have for us on Earth!#
Regarding your 2nd question, I cannot say with precision, but it is likely any such occurrences, if they happened, would have been at the solstices #see preceding text#.
Your last question is not quite clear. Do you mean its motion toward the constellation Hercules #point called 'solar apex'# at 70,000 km/hr? Or another?
You may also find the following previous answer to be of use:
And also this link:
The last includes a very detailed star map with lots of directional, positional info.
---------- FOLLOW-UP ----------
QUESTION: Dear Philip. Thank you very much for your time. I wonder if I could just check that I have understood you properly
I understand the basic astronomy of the solar system, including the inclination of the ecliptic plane to the equatorial plane and the consequent annual cycle of the celestial latitude of the Sun. I also understand that due to nutation the orientation of this inclination changes over time. Am I right in thinking that the ecliptic plane is roughly parallel to the galactic plane, and that the Sun's orbit of the galaxy lies in the latter plane?
Since the equinoctial points move in a full circle against the background of the 'fixed' stars once every 26,000 years, do you know when the equinoctial points would coincide with the points where the eclectic plane intersects the galactic plane?
Thanks again. I'm very grateful for your time.
Actually, this would not be so. As per the previous answer (again referencing the galactic map in Norton's Star Atlas# the position of the North Star #effectively defining the position of the North Celestial Pole and Earth's N. pole# is 27 degrees north of the galactic equator #i.e. galactic latitude 27 degrees#. If the galactic equator defines the galactic plane then this means an offset of #90- 27# = 63 degrees between the plane of the celestial equator and the galactic plane. However, the ecliptic is tilted as 23 1/2 degrees to the celestial equator, so that the offset of the ecliptic #ecliptic plane# to the galactic plane would be: #63 - 23.5# = 39.5 deg.
Also, as per the previous answer, and regarding the Sun's motion in the galaxy, then as seen from Earth the Sun is observed to be south of the galactic equator #or galactic plane# for half the year, and north of the galactic equator for the other half of the year. There is still that 39.5 deg offset in planes, so the Sun's 'orbit' cannot in the galactic plane. #Though it can 'briefly' intersect it via the path of its motion at one of two intersection points of the 2 planes, i.e. crossing north or south of the galactic plane.#
The equinoctial points are simply the two points at which the ecliptic intersects the celestial equator, defined at the vernal and autumnal equinoxes. Hence they are really two points, 180 degrees apart, on the celestial equator. As noted earlier, if the galactic equator defines the galactic plane then this means an offset of 63 degrees between the plane of the celestial equator and the galactic plane. Hence, we can visualize the ecliptic plane inclined to the galactic plane and intersecting it at a 39.5 degree angle.
Unfortunately, I cannot tell you *when* the equinoctial pts. will intersect the galactic plane. #A pity the Mayans couldn't have helped us out in the timing of that one!#