NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team (CC BY 4.0)
- Apoapsis
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The point of an orbit farthest from the central body, at distance a(1 + e). Around the Sun it is called aphelion, around Earth apogee.
Формулаr_a = a (1 + e)Див. також: Periapsis
- Argument of periapsis (ω)
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The angle, measured in the orbital plane from the ascending node, to the periapsis.
Формулаϖ = Ω + ωДе:
ϖlongitude of periapsis;Ωlongitude of the ascending nodeДив. також: Longitude of the ascending node (Ω), Periapsis
- Astronomical unit (au)
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A unit of length defined as exactly 149,597,870,700 metres, roughly the average distance between Earth and the Sun. Defined by IAU 2012 Resolution B2.
Див. також: Light-time, Light-year
- Axial tilt (obliquity)
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The angle between a body’s spin axis and the perpendicular to its orbital plane. Earth’s is about 23.44°, which causes the seasons. Uranus’s is about 98°.
Див. також: Declination, Solstice, Equinox
- Bortle scale
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A nine-class scale of night-sky darkness introduced by John E. Bortle in Sky & Telescope (2001), from class 1 (excellent dark site, stars to magnitude 7.6 to 8.0) to class 9 (inner city, magnitude 4.0 at best).
Див. також: Magnitude, Light pollution
- Circular orbital speed
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The speed needed for a circular orbit at distance r. Just above Earth’s surface it is about 7.9 km/s.
Формулаv_c = √(μ / r)Де:
μgravitational parameter;rdistance from the centreДив. також: Escape velocity, Vis-viva equation
- Constellation
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One of 88 officially defined regions of the sky (IAU, 1922 to 1930), named after a traditional star pattern such as Orion. Stars in a constellation are usually at very different distances.
Див. також: Magnitude
- Declination
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The angle of a point in the sky north (+) or south (−) of the celestial equator, the sky’s equivalent of latitude. The Sun’s declination swings between +23.44° and −23.44° over the year.
Формулаsin δ = sin ε · sin λДе:
δSun's declination;εobliquity of the ecliptic;λSun's ecliptic longitudeДив. також: Axial tilt (obliquity), Solstice
- Delta-v
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Change in velocity, in km/s. It is the currency of spaceflight: every manoeuvre costs delta-v, and a rocket’s propellant sets how much it has.
ФормулаΔv = v_e ln(m₀ / m_f)Де:
v_eeffective exhaust speed;m₀, m_finitial and final massДив. також: Hohmann transfer, Oberth effect
- Dwarf planet
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A body that orbits the Sun and is nearly round, but has not cleared its orbital neighbourhood and is not a moon (IAU 2006). Examples: Ceres, Pluto, Eris, Haumea, Makemake.
Див. також: Planet, Kuiper belt
- Eccentricity (e)
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How stretched an orbit is: 0 for a circle, between 0 and 1 for an ellipse, 1 for a parabola and above 1 for a hyperbola.
- Eclipse
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When one body passes into the shadow of another. Solar eclipse: the Moon’s shadow falls on Earth (at new Moon). Lunar eclipse: the Moon passes through Earth’s shadow (at full Moon).
Див. також: Node, Umbra and penumbra
- Ecliptic
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The plane of Earth’s orbit around the Sun, and the Sun’s apparent yearly path across the sky. It is the usual reference plane for orbits of solar-system bodies. It is tilted about 23.44° to Earth’s equator.
Див. також: Axial tilt (obliquity), Inclination (i), Node
- Ellipse
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A closed curve, the set of points whose distances to two fixed points (foci) add up to a constant. Bound orbits are ellipses with the central body at one focus (Kepler’s first law).
Формулаr = a(1 − e²) / (1 + e cos ν)Де:
asemi-major axis;eeccentricity;νtrue anomalyДив. також: Eccentricity (e), Semi-major axis (a)
- Equinox
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The moment when the Sun crosses the celestial equator (declination 0°), around 20 March and 22 September. Day and night are about equal everywhere.
Див. також: Solstice, Declination
- Escape velocity
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The minimum speed at which an unpowered body leaves a gravitating body for good. It is √2 times the circular speed at the same distance. Earth’s surface: 11.2 km/s.
Формулаv_esc = √(2μ / r)Див. також: Circular orbital speed, Specific orbital energy
- Free fall
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Motion under gravity alone. Astronauts in orbit feel weightless because they and their spacecraft are in free fall together, not because gravity is absent.
- Gravitational parameter (μ)
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The product of the gravitational constant and a body’s mass, μ = GM. It is measured far more precisely than G or M alone. Earth: 398,600.4 km³/s²; Sun: 1.32712 × 10¹¹ km³/s².
Формулаμ = G MДив. також: Gravity, Vis-viva equation
- Gravity
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The attraction between masses. In Newton’s description the force is proportional to both masses and inversely proportional to the square of their distance.
ФормулаF = G M m / r²Де:
G6.67430 × 10⁻¹¹ m³ kg⁻¹ s⁻² (CODATA 2018);M, mthe two masses;rdistance between centresДив. також: Gravitational parameter (μ), Free fall
- Hohmann transfer
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The minimum-energy two-burn transfer between two circular, coplanar orbits, along half of an ellipse touching both.
Формулаa_t = (r₁ + r₂) / 2Див. також: Delta-v, Vis-viva equation
- Inclination (i)
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The tilt of an orbital plane relative to a reference plane (the ecliptic for planets, the equator for satellites). Above 90° the orbit is retrograde.
Див. також: Longitude of the ascending node (Ω), Retrograde
- Kepler's third law
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The square of the orbital period is proportional to the cube of the semi-major axis. Around the Sun, with T in years and a in au, T² = a³.
ФормулаT² / a³ = 4π² / μДе:
Torbital period;asemi-major axis;μG(M + m)Див. також: Orbital period, Semi-major axis (a)
- Kuiper belt
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A region of icy bodies beyond Neptune, from about 30 to 50 au from the Sun. Pluto, Haumea and Makemake orbit there.
Див. також: Main asteroid belt, Dwarf planet
- Light pollution
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Excessive or misdirected artificial light, especially sky glow that hides stars. It also affects wildlife and wastes energy.
Див. також: Bortle scale
- Light-time
-
The time light needs to travel a given distance. Sunlight takes about 8 minutes 19 seconds to reach Earth. Commands to spacecraft are delayed by the light-time.
Формулаt = d / cДе:
ddistance;cspeed of light, 299,792.458 km/sДив. також: Astronomical unit (au)
- Light-year
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The distance light travels in vacuum in one Julian year (365.25 days): about 9.46 trillion km, or 63,241 au. It is a distance, not a time.
Формула1 ly = c × 365.25 × 86,400 sДе:
cspeed of light, 299,792.458 km/sДив. також: Astronomical unit (au), Light-time
- Longitude of the ascending node (Ω)
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The angle, measured in the reference plane from the reference direction, to the point where the orbit crosses the plane going north.
Див. також: Node, Inclination (i), Argument of periapsis (ω)
- Magnitude
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A logarithmic scale of brightness, where smaller numbers are brighter. Five magnitudes are exactly a factor of 100 in brightness.
Формулаm₁ − m₂ = −2.5 log₁₀(F₁ / F₂)Де:
mmagnitude;Fobserved fluxДив. також: Bortle scale
- Main asteroid belt
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The region between the orbits of Mars and Jupiter, roughly 2.2 to 3.3 au from the Sun, where most known asteroids orbit. Despite its reputation it is mostly empty space; its largest body is the dwarf planet Ceres.
Див. також: Kuiper belt, Dwarf planet
- Mean anomaly (M)
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An angle that increases uniformly with time, 360° per orbit. The actual position follows from Kepler’s equation.
ФормулаM = E − e sin EДе:
Eeccentric anomaly;eeccentricityДив. також: Eccentricity (e)
- Node
-
One of the two points where an orbit crosses a reference plane. The Moon’s nodes lie on the ecliptic; eclipses happen only when a new or full Moon occurs near a node.
Див. також: Longitude of the ascending node (Ω), Eclipse
- Oberth effect
-
A burn made where a spacecraft moves fastest (deep in a gravity well) changes its orbital energy the most. Departing from low orbit therefore costs less than the leftover speed you want far away.
ФормулаΔv = √(v∞² + v_esc²) − v_cДе:
v∞hyperbolic excess speed;v_escescape speed at the burn;v_ccircular speed at the burnДив. також: Delta-v, Escape velocity
- Orbit
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The path of a body moving under the gravity of another. In the two-body case it is a conic section: an ellipse (closed), a parabola or a hyperbola (open).
Див. також: Ellipse, Kepler's third law, Free fall
- Orbital period
-
The time taken to complete one orbit. The sidereal period is measured against the stars; the synodic period is measured relative to another moving body, such as the Sun seen from Earth.
ФормулаT = 2π √(a³/μ)Де:
asemi-major axis;μgravitational parameter of the central bodyДив. також: Kepler's third law, Synodic month
- Periapsis
-
The point of an orbit closest to the central body, at distance a(1 − e). Around the Sun it is called perihelion, around Earth perigee.
Формулаr_p = a (1 − e)Див. також: Apoapsis, Perihelion
- Perihelion
-
The point of an orbit around the Sun closest to the Sun. Earth passes it in early January at about 147.1 million km. The farthest point is aphelion.
- Planet
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By the 2006 IAU definition, a body that orbits the Sun, is massive enough to be nearly round under its own gravity, and has cleared the neighbourhood around its orbit. The solar system has eight.
Див. також: Dwarf planet, Orbit
- Retrograde
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Moving or rotating in the opposite direction to most bodies in the system. Venus rotates retrograde; Triton and Halley’s Comet orbit retrograde (inclination above 90°).
Див. також: Inclination (i), Solar day
- Semi-major axis (a)
-
Half of the longest diameter of an ellipse. It sets the orbit’s size, its energy and its period.
Формулаa = (r_p + r_a) / 2Де:
r_pperiapsis distance;r_aapoapsis distanceДив. також: Kepler's third law, Ellipse
- Sidereal day
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The time a body takes to rotate once relative to the distant stars. For Earth: 23 h 56 min 4.09 s.
Див. також: Solar day
- Sidereal month
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The time the Moon takes to orbit Earth once relative to the stars: 27.322 days.
Див. також: Synodic month
- Solar day
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The time from one noon to the next, i.e. one rotation relative to the Sun. It differs from the sidereal day because the body also moves along its orbit.
Формула1/P_solar = 1/P_rot − 1/P_orbДе:
P_rotsidereal rotation period (negative if retrograde);P_orborbital periodДив. також: Sidereal day, Retrograde
- Solstice
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The moment when the Sun reaches its farthest north (around 21 June) or south (around 21 December) declination. It gives the longest and shortest days.
Див. також: Equinox, Declination
- Specific orbital energy
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Orbital energy per unit mass. Negative for bound (elliptical) orbits, zero for parabolic escape, positive for hyperbolic.
Формулаε = v²/2 − μ/r = −μ / (2a)Див. також: Vis-viva equation, Escape velocity
- Synodic month
-
The time between two identical Moon phases, e.g. new Moon to new Moon: 29.531 days on average.
Формула1/P_syn = 1/P_sid − 1/P_yearДе:
P_sidsidereal month, 27.322 days;P_yearsidereal year, 365.256 daysДив. також: Sidereal month
- Umbra and penumbra
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The umbra is the dark central part of a shadow where the light source is completely hidden; the penumbra is the outer part where it is only partly hidden.
Див. також: Eclipse
- Vis-viva equation
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Gives the orbital speed at any distance from the orbit’s size alone. It expresses conservation of energy.
Формулаv = √(μ (2/r − 1/a))Де:
μgravitational parameter;rcurrent distance;asemi-major axisДив. також: Specific orbital energy, Circular orbital speed
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