Why don’t the Sun and Moon twinkle?
Though the Earth’s atmosphere distorts the light in all cases, the twinkling phenomenon is noticeable only when the object appears very small in the sky. Even planets and large stars may appear to twinkle only when they are near the horizon. The Sun and the Moon are obviously too large for the twinkling effect to be noticeable.
Does nearness of a celestial body to the horizon have any effect other than increasing the twinkling?
Yes. The colour of the object turns pink, a reason why the Sun appears red during sunrise or sunset. Mercury also sometimes appears pink when seen near the horizon.
What is meant by the apparent and true direction of the Sun?
The true direction of the Sun is always different from the apparent one due to refraction of Sun’s radiation in the Earth’s atmosphere.
What is the Celestial Sphere?
It is imaginary sphere with the Earth as its centre and having a very large radius. The position of bodies such as stars, planets and galaxies are specified by their coordinates on the celestial sphere. In ancient times, the astrologers considered the Earth to be surrounded by the Celestial Sphere were attached to it.
Does the celestial sphere concept have any significance in modern astronomy?
No. The Earth hides part of the celestial sphere below the horizon.
What are the celestial poles, the celestial equator and celestial meridians?
The celestial sphere is assumed to posses’ poles, equator and meridians like the Earth. The point on the celestial sphere directly above the Earth’s geographical North Pole is assumed to be the north celestial pole. Similarly, the point on the celestial sphere directly above the Earth’s geographical South Pole is assumed to be the south celestial pole.
What is the celestial equator?
The celestial equator is the imaginary line representing the circumference of the celestial sphere and situated midway between the celestial poles.
What is the significance of the celestial poles?
When the Earth rotates on its axis, the celestial sphere also appears to rotate around its axis. A star situated at a celestial pole directly above any of the Earth’s poles would appears not have any lateral movement while the Earth is rotating.