An orbit is continuous free fall. A satellite keeps moving sideways while gravity bends its path towards the planet. Begin with a circular orbit so that the radius and speed stay constant.
Explore gravitation and circular orbits →An object's physical size tells you how wide it is. Its angular size tells you how wide it looks from your position. A small nearby object can cover the same angle as a large distant object.
Explore angular size and radians →A telescope collects light and helps separate fine detail. Magnification is only one part of the story. Learn to distinguish aperture, focal length, magnification and resolution before using a formula.
Explore what a telescope actually changes →Two motions shape the sky: Earth spins, and objects also move in their orbits. Keep the observer's changing direction separate from the Moon's changing position around Earth.
Explore sky motion and moon phases →Many astronomy calculations become shorter when you compare two situations before inserting large numbers. Powers describe how a quantity scales; logarithms turn a multiplicative change into an additive one.
Explore ratios, scaling and logarithms →A star's luminosity describes the power it emits. The flux at an observer describes power arriving per unit area. Keeping these quantities separate explains why a powerful distant star can look faint.
Explore radiation and the inverse-square law →