A terminator or twilight zone is a moving line that divides the daylit side and the dark

night Night, or nighttime, is the period of darkness when the Sun is below the horizon. Sunlight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of ...

side of a

planetary body A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of planet, geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilib ...

. The terminator is defined as the locus of points on a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

where the line through the center of its parent

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

tangent In geometry, the tangent line (or simply tangent) to a plane curve at a given point is, intuitively, the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points o ...

. An observer on the terminator of such an orbiting body with an

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

would experience

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

due to

light scattering by particles Light scattering by particles is the process by which small particles (e.g. ice crystals, dust, atmospheric particulates, cosmic dust, and blood cells) light scattering, scatter light causing optical phenomena such as the Rayleigh scattering, blue ...

in the gaseous layer.

Earth's terminator

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, the terminator is a circle with a diameter that is approximately that of Earth. The terminator passes through any point on Earth's surface twice a day, at

sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...

and at

sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...

, apart from

polar region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...

s where this only occurs when the point is not experiencing

midnight sun Midnight sun, also known as polar day, is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When midnight sun is see ...

polar night Polar night is a phenomenon that occurs in the polar regions of Earth, northernmost and southernmost regions of Earth when the Sun remains below the horizon for more than 24 hours. This only occurs inside the polar circles. The opposite phen ...

. 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

eclipse An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...

s), the terminator path varies by time of day due to

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in progra ...

on its axis. The terminator path also varies by time of year due to

Earth's orbit Earth orbits the Sun at an astronomical unit, average distance of , or 8.317 light-second, light-minutes, in a retrograde and prograde motion, counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes & ...

al revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

during the

equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

es, and its maximum angle is approximately 23.5° to the pole during the

solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...

Surface transit speed

At the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately . 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).

Supersonic aircraft A supersonic aircraft is an aircraft capable of supersonic flight, that is, flying faster than the speed of sound (Mach number, Mach 1). Supersonic speed, Supersonic aircraft were developed in the second half of the twentieth century. Supersonic ...

jet fighter Fighter aircraft (early on also ''pursuit aircraft'') are military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the air ...

s or

Concorde Concorde () is a retired Anglo-French supersonic airliner jointly developed and manufactured by Sud Aviation and the British Aircraft Corporation (BAC). Studies started in 1954, and France and the United Kingdom signed a treaty establishin ...

and

Tupolev Tu-144 The Tupolev Tu-144 (; NATO reporting name: Charger) is a Soviet supersonic airliner, supersonic passenger airliner designed by Tupolev in operation from 1968 to 1999. The Tu-144 was the world's first commercial supersonic transport aircraft wit ...

supersonic transport The ogive.html" ;"title="Concorde supersonic transport had an ogive">ogival delta wing, a slender fuselage and four underslung Rolls-Royce/Snecma Olympus 593 engines. file:Tu-144.jpg, The Tupolev Tu-144 was the first SST to enter service and th ...

s 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

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

s, and it is possible to walk faster than the terminator at the poles, near to the

es. The visual effect is that of seeing the sun rise in the west, or set in the east.

Grey-line radio propagation

Strength of

radio propagation Radio propagation is the behavior of radio waves as they travel, or are wave propagation, propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio w ...

changes between day- and night-side of the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to 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 ...

. This is primarily because the

D layer The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to 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 ...

, which absorbs high frequency signals, disappears rapidly on the dark side of the terminator, whereas

the E E is the fifth letter of the Latin alphabet. E or e may also refer to: Computing and computation * E (1970s text editor), a text editor developed at the Stanford AI Lab in the 1970s * E (complexity), a set of decision problems solvable by a ...

and

F layer The F region of the ionosphere is home to the F layer of ionization, also called the Appleton–Barnett layer, after the English physicist Edward Appleton and New Zealand physicist and meteorologist Miles Barnett. As with other ionospheric secto ...

s above the

take longer to form.Adrian Weiss. (2011). ''Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer'', pp. 1–16, 1-22–1-24. Green Valley, AZ: Milliwatt QRP Books. This time-difference puts the

into a unique intermediate state along the terminator, called the "grey line".

Amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...

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 Antipode or Antipodes may refer to: Mathematics * Antipodal point, the diametrically opposite point on a circle or ''n''-sphere, also known as an antipode * Antipode, the convolution inverse of the identity on a Hopf algebra Geography * Antipodes ...

points.

Gallery

File:XEphem-sunset-animation.gif, A seasonal animation of the terminator line at sunset over central Europe. File:Mimas double terminator.jpg, The transition from light to dark takes place on two fronts in this image of Mimas. File:Earthterminator iss002 full.jpg, A photograph of part of the terminator crossing the surface of the Earth, as seen from the ISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced as

on the surface. File:Boznańska crater EW1023346761G.jpg, Boznańska crater near the terminator on Mercury

Lunar terminator

Timocharis crater terminator AS15-P-9423

The lunar terminator is the division between the illuminated and dark hemispheres of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. It is the lunar equivalent of the division between

and

day A day is the time rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...

on the Earth

spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface (mathematics), surface obtained by Surface of revolution, rotating an ellipse about one of its principal axes; in other words, an ellipsoid with t ...

, although the Moon's much lower rate of rotation means it takes longer for it to pass across the surface. At the equator, it moves at , as fast as an athletic human can run on earth. Due to the angle at which

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

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.

Lunar terminator illusion

The lunar terminator (or tilt) illusion is an

optical illusion In visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide varie ...

arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line

perpendicular In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

to the terminator) should correspond with the

position of the Sun The position of the Sun in the sky is a function of both the time and the geographic coordinate system, geographic location of observation on Earth's surface. As Earth's orbit, Earth orbits the Sun over the course of a year, the Sun appears to mo ...

, 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.

Scientific significance

Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an

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.

Low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...

satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from

, therefore their

solar cell A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

s are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of

Sun-synchronous orbit A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...

. 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.

References

External links

Current terminator

– Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)
The Moon Terminator Illusion (video)
{{authority control Earth phenomena Light Solar phenomena Parts of a day Articles containing video clips Lunar science