Timeline for How axial precession affects the night sky
Current License: CC BY-SA 4.0
9 events
| when toggle format | what | by | license | comment | |
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| Oct 10, 2023 at 17:00 | comment | added | AlexP | @Bedlasky: In ecliptic coordinates their latitude stays constant and their longiture progresses by about 1°23′48″ per century. In equatorial coordinates that varies from star to star, because it's a rotation around an oblique axis. Need to convert from equatorial to ecliptic coordinates at the start of the epoch, apply the progression of the longitude, then convert back to equatorial coordinates at the time of interest. Pay attention to the value of the obliquity used when doing the conversion -- it varies slowly for its own reasons. | |
| Oct 10, 2023 at 13:31 | comment | added | Bedlasky | Ok, but how about stars between? How much are they shifted? | |
| Oct 10, 2023 at 12:40 | comment | added | AlexP | @Bedlasky: It is shifted indeed, but it is not shifted towards the pole of the ecliptic. The distance between the axis of rotation and the axis of the ecliptic is always the same. The pole of the axis of rotation moves around the pole of the ecliptic, with the axis of rotation always inclinded 23° with respect to the axis of the ecliptic. The equator is always inclinded 23° with respect to the plane of the ecliptic. | |
| Oct 10, 2023 at 11:47 | comment | added | Bedlasky | I looked at that illustration and that basically confirm what I said. In a quarter of precession cycle, north pole is shifted by 23° from current position. So equator must be also shifted by 23°. So Sun is on the equator. | |
| Oct 10, 2023 at 9:12 | comment | added | AlexP | @Bedlasky: The oscillation between +23° (called the northern solstice) and −23° (called the southern solstice) happens in the course of one year. This is why in summer days are long and nights are short, and in winter days are short and nights are long; this is why some stars are visible in summer and not in winter, and vice-versa. And yes, midway between the solstices are two points, one in March and one in September, when the sun is at 0° with respect to the Equator. | |
| Oct 10, 2023 at 9:08 | comment | added | AlexP | @Bedlasky: Re-reading your comment... First, there is no privileged point of view. Second, the obliquity of the ecliptic does not change during a precession cycle. (Well, it does change, but only a little and this has nothing to do with precession.) Precession changes the direction in which the axis of rotation points, but it always keeps about the same inclination with respect to the ecliptic plane. Look at this illustration and notice that at any point in the cycle the north celestial pole is 23° away from the pole of the ecliptic. | |
| Oct 10, 2023 at 8:55 | comment | added | AlexP | @Bedlasky: I do not understand what you do not understand. You do understand that the sun makes a complete revolution with respect to the fixed stars in the course of one year? The obliquity of the ecliptic remains more or less constant, and anyway the changes in obliquity have nothing to do with precession. In any year there are two moments when the sun is above the equator, and they are called equinoxes. For the movement of the sun, the only thing precession does is that it makes a sidereal year about 20 minutes longer than a tropic year. | |
| Oct 10, 2023 at 4:26 | comment | added | Bedlasky | Thanks for detailed answer! But I am still confused. From observer's point of view, everything on the night sky is shifted by x° (x - axial tilt) as oppose to what would you see from space. Half way through axial precession cycle you see everything shifted by -x°. So in quarter of the cycle everything is shifted by 0°. So Sun is exactly on equator = no seasons. But apparently this don't happen - why? | |
| Oct 9, 2023 at 8:14 | history | answered | AlexP | CC BY-SA 4.0 |
