![]() The ecliptic is the horizontal orange line in the middle.ĥ, Sun moves along ecliptic to the left and completes its journey in one year. This image shows starry sky along ecliptic on with Sun and planets. ![]() Its length is approximately 23 hours and 56 minutesĤ, Sun moves relative to stars along a well defined path, called ecliptic. This is called culmination.ģ, Interval between two consecutive culminations of the star is reasonably constant and is called sidereal day. I adhere to terrestrial observer's point of view.ġ, Starry sky (and everything on it, including Sun, Moon, planets) rotates around a point near Polaris (north celestial pole)Ģ, A given star (and everything which doesn't move relative to stars) gets to its maximum height above horizon at south (by definition). When I speak about distances I mean (angular) distances in the sky, when i speak about position I mean position in the sky. ![]() This is not quite an answer but more than a simple comment. Images like this show the perspective of an observer outside the Earth, but for an observer fixed to the Earth, the Sun really moves (on average) 360 degrees in our sky in 24 hours. Which is more relevant for Earth-based timekeeping? Of course it’s the Sun, so we divide that 360 degrees by 24 hours. The reason the two are different is that we are moving with respect to the Sun more than we are moving with respect to the stars. (And it’s equally true that in 23h56m the Earth rotates 360 degrees with respect to the stars.) Put differently, in 24 hours the Earth rotates 360 degrees with respect to the Sun. If instead you ask, how long does it take to bring the Sun back to the same position, that is 24 hours. If you ask how long it takes to rotate enough to bring a given star back to the same position, that is 23 hours and 56 minutes. To answer the question “how long does it take the Earth to rotate once on its axis?” you have to first answer “rotate with respect to what reference point?” Here’s a slightly different way to think about it that might be helpful to you and/or the students.
0 Comments
Leave a Reply. |