The action of rotating about an axis or center
The dark central part of a sunspot.
a band of colors, as seen in a rainbow, produced by separation of the components of light by their different degrees of refraction according to wavelength.
The speed of something in a given direction.
the collection of eight planets and their moons in orbit around the sun, together with smaller bodies in the form of asteroids, meteoroids, and comets.
a celestial body orbiting the earth or another planet.
A tide just after the first or third quarters of the moon when there is least difference between high and low water.
a unit of measurement equal to 149.6 million kilometers, the mean distance from the center of the earth to the center of the sun
a space entirely devoid of matter.
a kind of radiation including visible light, radio waves, gamma rays, and X-rays, in which electric and magnetic fields vary simultaneously.
very weak gravity, as in an orbiting spacecraft.
an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified.
the scanning of the earth by satellite or high-flying aircraft in order to obtain information about it.
The first appearance of the new moon.
a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.