Rotation and revolution are types of circular motion that are distinguished by the location of the axis relative to the moving body. When the axis passes through the object, it rotates, but when the axis is outside the object, it revolves. Rotation is synonymous with spinning; revolution is synonymous with orbiting. The Earth rotates about its axis, which runs through the North and South Poles; at the same time, it revolves around the Sun.
Entries in astronomy (4)
A quadrant is an early navigational instrument used to measure the altitude of astronomical bodies or the angle between two objects. These measurements could be used to determine one’s latitude or position on a chart. A quadrant could be as simple as a quarter-circle shaped device (hence the name) with an arc graduated from 0° to 90°. Movable vanes, sighting devices called alidades, and plumbobs were incorporated to make operating the quadrant more convenient.
By the eighteenth century, many inventors were working on methods for determining longitude at sea. To do so, it was necessary to measure the angles of celestial objects while aboard a rocking ship with greater accuracy than could be achieved using a quadrant. The octant improved upon the design of the quadrant by adding a cleverly positioned pair of mirrors, a set of shades for observing bright objects, and a telescope. The mirrors a) enabled the observer to view both the horizon and the object to be sighted at once, reducing observational errors, and b) allowed the octant’s arc to shrink to 1/8 of a circle while retaining the ability to measure angles up to 90°. These upgrades created a measuring device that was smaller, more accurate, and could be used day or night.
Beginning in 1767, tables of the angle between the Moon and other celestial bodies were published. Longitude could also be calculated using this data, but some lunar distances were greater than 90°. To make navigation using the lunar method more practicable the sextant was invented. This was fundamentally similar to the octant, but covered 1/6 of a circle, or 60°, allowing for measurements up to 120°.
Mass is a measure of the amount of matter in an object, and remains constant regardless of where that object is (assuming it’s not moving very quickly indeed). Mass is often measured in grams or kilograms.
Weight is a measure of how strongly a large object, like a planet, pulls on a small object, like you. Weight is defined as the product of an object’s mass and the acceleration it experiences due to gravity. Weight is usually measured in pounds or newtons. An object’s weight varies depending on how far it is from the planet or other celestial body being considered, and how massive that planet is.
Of course, the reason that there is such widespread confusion about the difference between these concepts is that we almost always measure weights (or masses) on the Earth’s surface, where mass and weight can easily be converted by the simple expedient of multiplying by a constant. The upshot of this is that your mass is the same everywhere, but you would weigh less on the Moon than you do on Earth.
There’s often some confusion about what to call sub-asteroid sized objects that intersect with Earth’s orbit. It’s pretty straightforward: They’re called meteoroids when they’re in space, meteors when they’re streaking through the atmosphere, and meteorites if they manage to land without completely vaporizing.