A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. The terminator is defined as the locus of points on a planet or moon where the line through the center of its parent star is tangent. An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.
On Earth, the terminator is a circle with a diameter that is approximately that of Earth. [1] The terminator passes through any point on Earth's surface twice a day, at sunrise and at sunset, apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night. The circle separates the portion of Earth experiencing daylight from that experiencing darkness (night). While a little over one half of Earth is illuminated at any point in time (with exceptions during eclipses), the terminator path varies by time of day due to Earth's rotation on its axis. The terminator path also varies by time of year due to Earth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes, and its maximum angle is approximately 23.5° to the pole during the solstices. [2]
At the equator, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 metres per second (1,040 mph). This speed can appear to increase when near obstructions, such as the height of a mountain, as the shadow of the obstruction will be cast over the ground in advance of the terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness). [3]
Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic transports are the only aircraft able to overtake the maximum speed of the terminator at the equator. However, slower vehicles can overtake the terminator at higher latitudes, and it is possible to walk faster than the terminator at the poles, near to the equinoxes. The visual effect is that of seeing the sun rise in the west, or set in the east.
Strength of radio propagation changes between day- and night-side of the ionosphere. This is primarily because the D layer, which absorbs high frequency signals, disappears rapidly on the dark side of the terminator, whereas the E and F layers above the D layer take longer to form. [4] This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the "grey line". [5]
Amateur radio operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line" propagation, this signal path is a type of skywave propagation. Under good conditions, radio waves can travel along the terminator to antipodal points. [5]
The lunar terminator is the division between the illuminated and dark hemispheres of the Moon. [6] It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation [7] means it takes longer for it to pass across the surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth.
Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when the Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows provide accurate descriptions of the lunar terrain.
The lunar terminator (or tilt) illusion is an optical illusion arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line perpendicular to the terminator) should correspond with the position of the Sun, but does not appear to do so. The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based on planar geometry. [8] [9]
Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere can create a fuzzier terminator. As the particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the sun has set. Images showing a planetary terminator can be used to map topography: the position of the tip of a mountain behind the terminator line is measured when the Sun still or already illuminates it while the base of the mountain remains in shadow. [10]
Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse, therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit. This prolongs the operational life of a LEO satellite, as onboard battery life is prolonged. It also enables specific experiments that require minimum interference from the Sun, as the designers can opt to install the relevant sensors on the dark side of the satellite.
A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.
The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km (30 mi) to 965 km (600 mi) above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth. It also affects GPS signals that travel through this layer.
A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit.
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times Earth's diameter. The Moon always presents the same side to Earth, because gravitational pull has locked its rotation to the planet. This results in the lunar day of 29.5 Earth days matching the lunar month. The Moon's gravitational pull – and to a lesser extent the Sun's – are the main drivers of the tides.
A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. In common usage, the four major phases are the new moon, the first quarter, the full moon and the last quarter; the four minor phases are waxing crescent, waxing gibbous, waning gibbous, and waning crescent. A lunar month is the time between successive recurrences of the same phase: due to the eccentricity of the Moon's orbit, this duration is not perfectly constant but averages about 29.5 days.
The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.
Night is the period of ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact time when night begins and ends depends on the location and varies throughout the year, based on factors such as season and latitude.
In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.
The selenographic coordinate system is used to refer to locations on the surface of Earth's moon. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth. The longitude gives the position east or west of the Moon's prime meridian, which is the line passing from the lunar north pole through the point on the lunar surface directly facing Earth to the lunar south pole. This can be thought of as the midpoint of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar equator. Both of these coordinates are given in degrees.
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
A peak of eternal light (PEL) is a hypothetical point on the surface of an astronomical body that is always in sunlight. Such a peak must have high latitude, high elevation, and be on a body with very small axial tilt. The existence of such peaks was first postulated by Beer and Mädler in 1837. The pair said about the lunar polar mountains: "...many of these peaks have eternal sunshine". These polar peaks were later mentioned by Camille Flammarion in 1879, who speculated that there may exist pics de lumière éternelle at the poles of the Moon. PELs would be advantageous for space exploration and colonization due to the ability of an electrical device located there to receive solar power regardless of the time of day or day of the year, and the relatively stable temperature range.
Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources were removed from the night sky, only direct starlight would be visible.
The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a situation known as tidal locking.
The Moon is the largest natural satellite of and the closest major astronomical object to Earth. The Moon may be observed by using a variety of optical instruments, ranging from the naked eye to large telescopes. The Moon is the only celestial body upon which surface features can be discerned with the unaided eyes of most people.
Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, but this article primarily discusses daytime on Earth.
Earth's shadow is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period, the shadow's visible fringe – sometimes called the dark segment or twilight wedge – appears as a dark and diffuse band just above the horizon, most distinct when the sky is clear.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
Solar eclipses on the Moon are caused when the planet Earth passes in front of the Sun and blocks its light. Viewers on Earth experience a lunar eclipse during a solar eclipse on the Moon.
Long-distance observation is any visual observation, for sightseeing or photography, that targets all the objects, visible from the extremal distance with the possibility to see them closely. The long-distance observations can't cover:
The roughly spherical shape of Earth can be empirically evidenced by many different types of observation, ranging from ground level, flight, or orbit. The spherical shape causes a number of effects and phenomena that combined disprove flat Earth beliefs. These include the visibility of distant objects on Earth's surface; lunar eclipses; appearance of the Moon; observation of the sky from altitude; observation of certain fixed stars from different locations; observing the Sun; surface navigation; grid distortion on a spherical surface; weather systems; gravity; and modern technology.
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