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A

Apoapsis

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.

Formula r_a = a (1 + e)

Lihat uga: Periapsis

Argument of periapsis (ω)

The angle, measured in the orbital plane from the ascending node, to the periapsis.

Formula ϖ = Ω + ω

Kapan: ϖ longitude of periapsis; Ω longitude of the ascending node

Lihat uga: Longitude of the ascending node (Ω), Periapsis

Astronomical unit (au)

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.

Lihat uga: Light-time, Light-year

Axial tilt (obliquity)

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°.

Lihat uga: Declination, Solstice, Equinox

B

Bortle scale

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).

Lihat uga: Magnitude, Light pollution

C

Circular orbital speed

The speed needed for a circular orbit at distance r. Just above Earth’s surface it is about 7.9 km/s.

Formula v_c = √(μ / r)

Kapan: μ gravitational parameter; r distance from the centre

Lihat uga: Escape velocity, Vis-viva equation

Constellation

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.

Lihat uga: Magnitude

D

Declination

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.

Formula sin δ = sin ε · sin λ

Kapan: δ Sun's declination; ε obliquity of the ecliptic; λ Sun's ecliptic longitude

Lihat uga: Axial tilt (obliquity), Solstice

Delta-v

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.

Formula Δv = v_e ln(m₀ / m_f)

Kapan: v_e effective exhaust speed; m₀, m_f initial and final mass

Lihat uga: Hohmann transfer, Oberth effect

Dwarf planet

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.

Lihat uga: Planet, Kuiper belt

E

Eccentricity (e)

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.

Lihat uga: Ellipse, Periapsis

Eclipse

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).

Lihat uga: Node, Umbra and penumbra

Ecliptic

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.

Lihat uga: Axial tilt (obliquity), Inclination (i), Node

Ellipse

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).

Formula r = a(1 − e²) / (1 + e cos ν)

Kapan: a semi-major axis; e eccentricity; ν true anomaly

Lihat uga: Eccentricity (e), Semi-major axis (a)

Equinox

The moment when the Sun crosses the celestial equator (declination 0°), around 20 March and 22 September. Day and night are about equal everywhere.

Lihat uga: Solstice, Declination

Escape velocity

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.

Formula v_esc = √(2μ / r)

Lihat uga: Circular orbital speed, Specific orbital energy

F

Free fall

Motion under gravity alone. Astronauts in orbit feel weightless because they and their spacecraft are in free fall together, not because gravity is absent.

Lihat uga: Orbit, Gravity

G

Gravitational parameter (μ)

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.

Formula μ = G M

Lihat uga: Gravity, Vis-viva equation

Gravity

The attraction between masses. In Newton’s description the force is proportional to both masses and inversely proportional to the square of their distance.

Formula F = G M m / r²

Kapan: G 6.67430 × 10⁻¹¹ m³ kg⁻¹ s⁻² (CODATA 2018); M, m the two masses; r distance between centres

Lihat uga: Gravitational parameter (μ), Free fall

H

Hohmann transfer

The minimum-energy two-burn transfer between two circular, coplanar orbits, along half of an ellipse touching both.

Formula a_t = (r₁ + r₂) / 2

Lihat uga: Delta-v, Vis-viva equation

I

Inclination (i)

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.

Lihat uga: Longitude of the ascending node (Ω), Retrograde

K

Kepler's third law

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.

Formula T² / a³ = 4π² / μ

Kapan: T orbital period; a semi-major axis; μ G(M + m)

Lihat uga: Orbital period, Semi-major axis (a)

Kuiper belt

A region of icy bodies beyond Neptune, from about 30 to 50 au from the Sun. Pluto, Haumea and Makemake orbit there.

Lihat uga: Main asteroid belt, Dwarf planet

L

Light pollution

Excessive or misdirected artificial light, especially sky glow that hides stars. It also affects wildlife and wastes energy.

Lihat uga: 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.

Formula t = d / c

Kapan: d distance; c speed of light, 299,792.458 km/s

Lihat uga: Astronomical unit (au)

Light-year

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.

Formula 1 ly = c × 365.25 × 86,400 s

Kapan: c speed of light, 299,792.458 km/s

Lihat uga: Astronomical unit (au), Light-time

Longitude of the ascending node (Ω)

The angle, measured in the reference plane from the reference direction, to the point where the orbit crosses the plane going north.

Lihat uga: Node, Inclination (i), Argument of periapsis (ω)

M

Magnitude

A logarithmic scale of brightness, where smaller numbers are brighter. Five magnitudes are exactly a factor of 100 in brightness.

Formula m₁ − m₂ = −2.5 log₁₀(F₁ / F₂)

Kapan: m magnitude; F observed flux

Lihat uga: Bortle scale

Main asteroid belt

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.

Lihat uga: Kuiper belt, Dwarf planet

Mean anomaly (M)

An angle that increases uniformly with time, 360° per orbit. The actual position follows from Kepler’s equation.

Formula M = E − e sin E

Kapan: E eccentric anomaly; e eccentricity

Lihat uga: Eccentricity (e)

N

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.

Lihat uga: Longitude of the ascending node (Ω), Eclipse

O

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.

Formula Δv = √(v∞² + v_esc²) − v_c

Kapan: v∞ hyperbolic excess speed; v_esc escape speed at the burn; v_c circular speed at the burn

Lihat uga: Delta-v, Escape velocity

Orbit

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).

Lihat uga: 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.

Formula T = 2π √(a³/μ)

Kapan: a semi-major axis; μ gravitational parameter of the central body

Lihat uga: Kepler's third law, Synodic month

P

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.

Formula r_p = a (1 − e)

Lihat uga: 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.

Lihat uga: Periapsis, Apoapsis

Planet

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.

Lihat uga: Dwarf planet, Orbit

R

Retrograde

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°).

Lihat uga: Inclination (i), Solar day

S

Semi-major axis (a)

Half of the longest diameter of an ellipse. It sets the orbit’s size, its energy and its period.

Formula a = (r_p + r_a) / 2

Kapan: r_p periapsis distance; r_a apoapsis distance

Lihat uga: Kepler's third law, Ellipse

Sidereal day

The time a body takes to rotate once relative to the distant stars. For Earth: 23 h 56 min 4.09 s.

Lihat uga: Solar day

Sidereal month

The time the Moon takes to orbit Earth once relative to the stars: 27.322 days.

Lihat uga: Synodic month

Solar day

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.

Formula 1/P_solar = 1/P_rot − 1/P_orb

Kapan: P_rot sidereal rotation period (negative if retrograde); P_orb orbital period

Lihat uga: Sidereal day, Retrograde

Solstice

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.

Lihat uga: Equinox, Declination

Specific orbital energy

Orbital energy per unit mass. Negative for bound (elliptical) orbits, zero for parabolic escape, positive for hyperbolic.

Formula ε = v²/2 − μ/r = −μ / (2a)

Lihat uga: 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.

Formula 1/P_syn = 1/P_sid − 1/P_year

Kapan: P_sid sidereal month, 27.322 days; P_year sidereal year, 365.256 days

Lihat uga: Sidereal month

U

Umbra and penumbra

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.

Lihat uga: Eclipse

V

Vis-viva equation

Gives the orbital speed at any distance from the orbit’s size alone. It expresses conservation of energy.

Formula v = √(μ (2/r − 1/a))

Kapan: μ gravitational parameter; r current distance; a semi-major axis

Lihat uga: Specific orbital energy, Circular orbital speed

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