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.
ፎርሙላr_a = a (1 + e)እሺ: Periapsis
- Argument of periapsis (ω)
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The angle, measured in the orbital plane from the ascending node, to the periapsis.
ፎርሙላϖ = Ω + ωየት:
ϖlongitude of periapsis;Ωlongitude of the ascending node - 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.
እሺ: 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°.
እሺ: 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).
እሺ: 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.
ፎርሙላv_c = √(μ / r)የት:
μgravitational parameter;rdistance from the centre - 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.
እሺ: 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.
ፎርሙላsin δ = sin ε · sin λየት:
δSun's declination;εobliquity of the ecliptic;λSun's ecliptic longitude - 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.
ፎርሙላΔv = v_e ln(m₀ / m_f)የት:
v_eeffective exhaust speed;m₀, m_finitial and final mass - 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.
እሺ: 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).
እሺ: 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.
- 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).
ፎርሙላr = a(1 − e²) / (1 + e cos ν)የት:
asemi-major axis;eeccentricity;νtrue anomaly - 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.
እሺ: 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.
ፎርሙላv_esc = √(2μ / r) - 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.
ፎርሙላμ = G Mእሺ: 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.
ፎርሙላF = G M m / r²የት:
G6.67430 × 10⁻¹¹ m³ kg⁻¹ s⁻² (CODATA 2018);M, mthe two masses;rdistance between centres - Hohmann transfer
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The minimum-energy two-burn transfer between two circular, coplanar orbits, along half of an ellipse touching both.
ፎርሙላa_t = (r₁ + r₂) / 2እሺ: 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.
- 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.
ፎርሙላT² / a³ = 4π² / μየት:
Torbital period;asemi-major axis;μG(M + m) - 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.
- Light pollution
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Excessive or misdirected artificial light, especially sky glow that hides stars. It also affects wildlife and wastes energy.
እሺ: 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.
ፎርሙላt = d / cየት:
ddistance;cspeed of light, 299,792.458 km/s - 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.
ፎርሙላ1 ly = c × 365.25 × 86,400 sየት:
cspeed of light, 299,792.458 km/s - 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.
- 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.
ፎርሙላm₁ − m₂ = −2.5 log₁₀(F₁ / F₂)የት:
mmagnitude;Fobserved fluxእሺ: 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.
እሺ: 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.
ፎርሙላM = E − e sin Eየት:
Eeccentric anomaly;eeccentricityእሺ: 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.
- 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.
ፎርሙላΔv = √(v∞² + v_esc²) − v_cየት:
v∞hyperbolic excess speed;v_escescape speed at the burn;v_ccircular speed at the burnእሺ: 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).
- 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.
ፎርሙላT = 2π √(a³/μ)የት:
asemi-major axis;μgravitational parameter of the central body - Periapsis
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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.
ፎርሙላr_p = a (1 − e)እሺ: 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.
እሺ: 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°).
እሺ: 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.
ፎርሙላa = (r_p + r_a) / 2የት:
r_pperiapsis distance;r_aapoapsis distance - 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.
እሺ: Solar day
- Sidereal month
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The time the Moon takes to orbit Earth once relative to the stars: 27.322 days.
እሺ: 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.
ፎርሙላ1/P_solar = 1/P_rot − 1/P_orbየት:
P_rotsidereal rotation period (negative if retrograde);P_orborbital periodእሺ: 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.
እሺ: 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.
ፎርሙላε = v²/2 − μ/r = −μ / (2a) - 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.
ፎርሙላ1/P_syn = 1/P_sid − 1/P_yearየት:
P_sidsidereal month, 27.322 days;P_yearsidereal year, 365.256 daysእሺ: 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.
እሺ: 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.
ፎርሙላv = √(μ (2/r − 1/a))የት:
μgravitational parameter;rcurrent distance;asemi-major axis
የጥናት መሳሪያዎች እና ትምህርቶች ለመማር ብቻ ቀላል ሞዴሎችን ይጠቀማሉ. ለመሻገር፣ ለሚሽን ዕቅድ ወይም ለየትኛውም ኦፕሬሽናል ውሳኔ ሊውሉ አይገባም. የምስክር ወረቀቶች ነፃ ናቸው እና ያልተጠበቁ ናቸው። የሥልጠና ይዘት ማረጋገጫን አንብብ