What are annular eclipses? What will be an ideal response? ...

[madam-professor]

What are annular eclipses?
 
  What will be an ideal response?

Question 2

Describe flux.
 
  What will be an ideal response?

[shayla]

Answer to Question 1

Sometimes when the Moon crosses between Earth and the Sun, it is a little closer to Earth and thus too small to fully cover the Sun. In such a case, you would witness an annular eclipse. That is a solar eclipse in which an annulus (meaning ring) of the Sun's disk is visible around the disk of the Moon. The eclipse never becomes total; it never quite gets dark, and you can't see the faint features of the solar atmosphere. Annular eclipses occur because the Moon follows a slightly elliptical orbit around Earth. If the Moon is in the farther part of its orbit during totality, its apparent diameter will be less than the apparent diameter of the Sun, and thus you see an annular eclipse. Furthermore, Earth's orbit is slightly elliptical, so the Earth- to-Sun distance varies slightly, and consequently so does the apparent diameter of the solar disk, contributing to the effect of the Moon's varying apparent size.

Answer to Question 2

Brightness is quite subjective, depending on both the physiology of eyes and the psychology of perception. To be scientifically accurate, you should refer to flux-a measure of the light energy from a star that hits one square meter in one second. With modern scientific instruments, astronomers can measure the flux of starlight with high precision and then use a simple mathematical relationship that relates light flux to apparent visual magnitude. Instead of saying that the star known by the charming name Chort (Theta Leonis) is about third magnitude, you can say its magnitude is 3.34. Thus, precise modern measurements of the brightness of stars are still connected to observations of apparent visual magnitude that go back to the time of Hipparchus.