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.
Công thứcr_a = a (1 + e)Xem thêm: Periapsis
- Argument of periapsis (ω)
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The angle, measured in the orbital plane from the ascending node, to the periapsis.
Công thứcϖ = Ω + ωỞ đâu:
ϖlongitude of periapsis;Ωlongitude of the ascending nodeXem thêm: 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.
Xem thêm: 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°.
Xem thêm: 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).
Xem thêm: 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.
Công thứcv_c = √(μ / r)Ở đâu:
μgravitational parameter;rdistance from the centreXem thêm: 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.
Xem thêm: 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.
Công thứcsin δ = sin ε · sin λỞ đâu:
δSun's declination;εobliquity of the ecliptic;λSun's ecliptic longitudeXem thêm: 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.
Công thứcΔv = v_e ln(m₀ / m_f)Ở đâu:
v_eeffective exhaust speed;m₀, m_finitial and final massXem thêm: 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.
Xem thêm: 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).
Xem thêm: 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.
Xem thêm: 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).
Công thứcr = a(1 − e²) / (1 + e cos ν)Ở đâu:
asemi-major axis;eeccentricity;νtrue anomalyXem thêm: 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.
Xem thêm: 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.
Công thứcv_esc = √(2μ / r)Xem thêm: 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.
Công thứcμ = G MXem thêm: 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.
Công thứcF = G M m / r²Ở đâu:
G6.67430 × 10⁻¹¹ m³ kg⁻¹ s⁻² (CODATA 2018);M, mthe two masses;rdistance between centresXem thêm: 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.
Công thứca_t = (r₁ + r₂) / 2Xem thêm: 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.
Xem thêm: 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.
Công thứcT² / a³ = 4π² / μỞ đâu:
Torbital period;asemi-major axis;μG(M + m)Xem thêm: 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.
Xem thêm: 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.
Xem thêm: Bortle scale
- Light-time
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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.
Công thứct = d / cỞ đâu:
ddistance;cspeed of light, 299,792.458 km/sXem thêm: 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.
Công thức1 ly = c × 365.25 × 86,400 sỞ đâu:
cspeed of light, 299,792.458 km/sXem thêm: 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.
Xem thêm: 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.
Công thứcm₁ − m₂ = −2.5 log₁₀(F₁ / F₂)Ở đâu:
mmagnitude;Fobserved fluxXem thêm: 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.
Xem thêm: 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.
Công thứcM = E − e sin EỞ đâu:
Eeccentric anomaly;eeccentricityXem thêm: Eccentricity (e)
- Node
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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.
Xem thêm: Longitude of the ascending node (Ω), Eclipse
- Oberth effect
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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.
Công thứcΔv = √(v∞² + v_esc²) − v_cỞ đâu:
v∞hyperbolic excess speed;v_escescape speed at the burn;v_ccircular speed at the burnXem thêm: 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).
Xem thêm: Ellipse, Kepler's third law, Free fall
- Orbital period
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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.
Công thứcT = 2π √(a³/μ)Ở đâu:
asemi-major axis;μgravitational parameter of the central bodyXem thêm: 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.
Công thứcr_p = a (1 − e)Xem thêm: Apoapsis, Perihelion
- Perihelion
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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.
Xem thêm: 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°).
Xem thêm: Inclination (i), Solar day
- Semi-major axis (a)
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Half of the longest diameter of an ellipse. It sets the orbit’s size, its energy and its period.
Công thứca = (r_p + r_a) / 2Ở đâu:
r_pperiapsis distance;r_aapoapsis distanceXem thêm: 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.
Xem thêm: Solar day
- Sidereal month
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The time the Moon takes to orbit Earth once relative to the stars: 27.322 days.
Xem thêm: 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.
Công thức1/P_solar = 1/P_rot − 1/P_orbỞ đâu:
P_rotsidereal rotation period (negative if retrograde);P_orborbital periodXem thêm: 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.
Xem thêm: 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.
Công thứcε = v²/2 − μ/r = −μ / (2a)Xem thêm: Vis-viva equation, Escape velocity
- Synodic month
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The time between two identical Moon phases, e.g. new Moon to new Moon: 29.531 days on average.
Công thức1/P_syn = 1/P_sid − 1/P_yearỞ đâu:
P_sidsidereal month, 27.322 days;P_yearsidereal year, 365.256 daysXem thêm: 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.
Xem thêm: 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.
Công thứcv = √(μ (2/r − 1/a))Ở đâu:
μgravitational parameter;rcurrent distance;asemi-major axisXem thêm: Specific orbital energy, Circular orbital speed
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