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Moon
Mean Radius
1,079 mi
Average Distance from Earth
238,855 mi
Surface Gravity
5.32 ft/s²
Orbital Period
27.32 days
Definition
Scientific Concept
sun
Average Distance from Earth
92,955,807 miles
Diameter
864,337 miles
Surface Temperature
9,941 Fahrenheit
Core Temperature
27,000,000 Fahrenheit
Definition
Scientific Concept
Concepts & Definitions
Analemma
— A diagram showing the position of the Sun in the sky as seen from a fixed location at the same mean solar time every day.
Angular Diameter
— The visual size of an object as seen from a specific point, measured in degrees or radians.
Angular Resolution
— The ability of an optical instrument or the eye to distinguish small details of an object.
Angular Speed
— The rate at which an object changes its angle, relative to a specific point, over a period of time.
Antarctica
— Earth's southernmost continent, containing the geographic South Pole and situated in the Antarctic region.
Atmospheric Compression
— A hypothetical optical phenomenon where the density of the air near the surface causes the base of distant objects to appear to shrink or vanish.
Atmospheric Extinction
— The reduction in the intensity of light from a celestial body as it passes through the Earth's atmosphere.
Atmospheric Refraction
— The bending of light or other electromagnetic waves as they pass through layers of air with varying densities.
Axial Tilt
— The angle between an object's rotational axis and its orbital axis, which causes seasonal variations in temperature and daylight.
Azimuth
— The direction of a celestial object from the observer, expressed as the angular distance from the north or south point of the horizon.
Bedford Level Experiment
— A series of 19th-century observations performed on a six-mile stretch of the Old Bedford River to test the Earth's shape.
Bottom-up Vanishing
— The visual phenomenon where the lower portion of a distant object is obscured by the horizon before the upper portion.
Celestial Equator
— The great circle on the imaginary celestial sphere in the same plane as the Earth's equator.
Celestial Navigation
— The art and science of finding one's geographic position by means of observations of the sun, moon, planets, and stars.
Celestial Pole
— Two imaginary points in the sky directly above the Earth's North and South poles around which stars appear to revolve.
Celestial Sphere
— An imaginary rotating sphere of gigantic radius with the Earth at its center, used to map the positions of stars.
Centrifugal Force
— An apparent force that acts outward on a body moving around a center, arising from the body's inertia.
Circumnavigation
— The action or process of sailing or otherwise traveling all the way around something, especially the world.
Circumpolar Stars
— Stars that, as viewed from a given latitude on Earth, never set below the horizon due to their proximity to one of the celestial poles.
Convergence
— The visual phenomenon where parallel lines or surfaces appear to move closer together as they recede into the distance.
Coriolis Effect
— The apparent deflection of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere due to Earth's rotation.
Crepuscular Rays
— Rays of sunlight that appear to radiate from a single point in the sky, usually during twilight, caused by shadows of clouds or mountains.
Curvature
— The degree to which a geometric object, such as a surface, deviates from being flat.
Day-Night Cycle
— The sequence of light and darkness caused by the rotation of a planet as it faces toward and away from the Sun.
Density and Buoyancy
— Physical properties used in flat earth arguments to explain the downward acceleration of objects based on their relative mass and the medium surrounding them.
Diffraction Limit
— The minimum angular separation between two sources that can be distinguished by an optical instrument.
Direct Observation
— Evidence collected through the senses or first-hand experience without intermediate inference or interpretation.
Earth's Radius
— The distance from the center of the Earth to its surface, approximately 6,371 kilometers, used as a constant in geodetic and navigation calculations.
Empirical Evidence
— Information acquired by means of observation or experimentation that is used to validate a hypothesis.
Equator
— The imaginary line around the middle of the Earth, equidistant from the North and South Poles, dividing the planet into the Northern and Southern Hemispheres.
Equinox
— The time or date twice each year at which the sun crosses the celestial equator, when day and night are of equal length.
Equipotential Surface
— A surface over which the gravitational potential is constant, meaning water will rest parallel to it.
Eratosthenes
— An ancient Greek mathematician and geographer who famously calculated the Earth's circumference using the angles of shadows in different locations.
Fata Morgana
— A complex form of superior mirage that is seen in a narrow band just above the horizon, significantly distorting the object on which it is based.
Firmament
— A theoretical physical dome or vault over the Earth that separates the atmosphere from the heavens.
Fluid Statics
— The branch of fluid mechanics that studies fluids at rest and the pressure exerted by a fluid or by a body immersed in a fluid.
Foucault Pendulum
— A simple device consisting of a tall pendulum free to swing in any vertical plane, which shows the rotation of the Earth.
Geocentrism
— The belief or model that the Earth is the stationary center of the universe, with all celestial bodies orbiting it.
Geodesy
— The science of accurately measuring and understanding Earth's geometric shape, orientation in space, and gravity field.
Geodetic Datum
— A coordinate system and set of reference points used to locate places on the Earth and define its size and shape for mapping.
Geoid
— The hypothetical shape of the earth, coinciding with mean sea level and its imagined extension under land clusters.
Geometric Curvature
— The theoretical physical curve of a sphere's surface determined by pure geometry without environmental or optical distortions.
Geometric Horizon
— The theoretical line of sight that is perfectly tangent to the surface of a sphere.
Gleason's Map
— An azimuthal equidistant map projection centered on the North Pole, often used to represent flat Earth geography.
Glitter Path
— A long, narrow track of specular reflection of sunlight or moonlight on a disturbed water surface.
Gravity
— The universal force of attraction acting between all matter, proportional to mass and inversely proportional to the square of the distance.
Great Circle Route
— The shortest distance between two points on the surface of a sphere, representing a section of a circle whose center is the center of the sphere.
Heliocentrism
— The astronomical model in which the Earth and planets revolve around the Sun at the center of the universe or solar system.
Hidden Height
— The portion of a distant object that is obstructed from view by the physical curvature of the Earth.
High-Altitude Ballooning
— The use of unmanned or manned balloons to reach the stratosphere for scientific observation or photography.
High-Optical Zoom
— The use of physical lens adjustment to magnify a distant image without losing the detail inherent in the light hitting the sensor.
Horizon Dip
— The angle between a horizontal line at the observer's eye and the line of sight to the apparent horizon.
Hydrostatics
— The branch of physics that deals with the characteristics of fluids at rest and the pressure they exert.
Ice Wall
— A hypothetical massive barrier of ice located at the outer perimeter of a flat Earth model, often identified with the coast of Antarctica.
Inferior Mirage
— A mirage in which the image of an object appears below the actual object, often seen as a 'puddle' on hot roads or at the horizon over water.
Latitude
— A geographic coordinate that specifies the north-south position of a point on the Earth's surface, measured in degrees from the equator.
Law of Perspective
— A mathematical system for representing three-dimensional objects on a two-dimensional surface so that they appear to recede into the distance.
Level of Eye
— A horizontal plane passing through the observer's eyes, used as a reference point in perspective and navigation.
Lunar Eclipse
— An astronomical event that occurs when the Moon moves into the Earth's shadow, preventing sunlight from reaching the lunar surface.
Magnification
— The process of enlarging the apparent size, not physical size, of something, typically through the use of optical lenses.
Oblate Spheroid
— The shape of a sphere that is slightly flattened at the poles and bulging at the equator.
Obstruction
— The state of being blocked or hidden from view by a physical mass, such as the curve of the horizon.
Optical Horizon
— The visible boundary between the Earth and the sky as affected by atmospheric refraction and local topography.
Penumbra
— The partially shaded outer region of the shadow cast by an opaque object, such as the moon during a solar eclipse.
Perspective Distortion
— A warping or transformation of an object and its surrounding area that differs from what the object would look like with a normal focal length.
Polaris
— Also known as the North Star, it is a bright star located nearly at the north celestial pole around which the northern sky appears to rotate.
Rayleigh Scattering
— The scattering of light by particles in the atmosphere, which is more intense at shorter wavelengths and changes the sky's color.
Refraction Gradient
— The rate of change in the refractive index of the atmosphere with respect to altitude.
Refractive Index
— A dimensionless number that describes how fast light travels through a medium relative to a vacuum.
Scientific Method
— A systematic process for experimentation and observation used to explore observations and answer questions.
Solar Declination
— The angle between the rays of the Sun and the plane of the Earth's equator.
Solar Eclipse
— An astronomical event that occurs when the Moon passes between the Sun and Earth, blocking all or part of the Sun's light.
Solar Insolation
— The amount of solar radiation reaching a given area, varying by latitude and time of year.
Solar Noon
— The time at which the sun is at its highest point in the sky for a specific observer.
Solstice
— Either of the two times in the year when the Sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days.
Southern Cross
— A prominent constellation in the Southern Hemisphere used for navigation to find the direction of the South Pole.
Specular Reflection
— A mirror-like reflection of light from a surface, where light from a single incoming direction is reflected into a single outgoing direction.
Spherical Trigonometry
— A branch of geometry that deals with the relationships between trigonometric functions of the sides and angles of the polygons limited by great circles on a sphere.
Standard Refraction
— The average value of atmospheric refraction used in navigation and surveying, typically allowing observers to see slightly further than the geometric horizon.
Static Equilibrium
— A state in which a physical system, such as a body of water, is at rest and the net force acting on it is zero.
Stellar Parallax
— The apparent shift in position of a nearby star against a background of distant stars due to Earth's orbit around the Sun.
Stratosphere
— The second major layer of Earth's atmosphere, located above the troposphere and containing the ozone layer.
Subsolar Point
— The point on a planet where its sun is perceived to be directly overhead (at the zenith).
Superior Mirage
— An atmospheric optical effect where an image appears above the actual position of the object due to a temperature inversion.
Syzygy
— The nearly straight-line configuration of three celestial bodies in a gravitational system, such as the Sun, Moon, and Earth during an eclipse.
Theodolite
— A precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes.
Tropic of Capricorn
— The circle of latitude that contains the subsolar point on the December solstice, representing the southernmost latitude where the sun can be directly overhead.
Umbra
— The fully shaded inner region of a shadow cast by an opaque object, such as the Moon during a solar eclipse.
Vanishing Point
— The point in a perspective drawing or a field of vision at which parallel lines appear to meet.
Zenith
— The point in the celestial sphere directly above an observer.
Zetetic Method
— Samuel Rowbotham (Parallax), Bedford Level Experiment, Atmospheric Refraction, and the 8-inch-per-mile-squared curvature rule.
