Glossario

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)

Vedi anche: Periapsis

Argument of periapsis (ω)

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

Formula ϖ = Ω + ω

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

Vedi anche: 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.

Vedi anche: 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°.

Vedi anche: 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).

Vedi anche: 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)

Dove: μ gravitational parameter; r distance from the centre

Vedi anche: 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.

Vedi anche: 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

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

Vedi anche: 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)

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

Vedi anche: 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.

Vedi anche: 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.

Vedi anche: 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).

Vedi anche: 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.

Vedi anche: 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)

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

Vedi anche: 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.

Vedi anche: 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)

Vedi anche: 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.

Vedi anche: 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

Vedi anche: 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

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

Vedi anche: 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

Vedi anche: 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.

Vedi anche: 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

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

Vedi anche: 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.

Vedi anche: 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.

Vedi anche: 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

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

Vedi anche: 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

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

Vedi anche: 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.

Vedi anche: 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)

Dove: m magnitude; F observed flux

Vedi anche: 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.

Vedi anche: 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

Dove: E eccentric anomaly; e eccentricity

Vedi anche: 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.

Vedi anche: 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

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

Vedi anche: 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).

Vedi anche: 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)

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

Vedi anche: 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)

Vedi anche: 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.

Vedi anche: 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.

Vedi anche: 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°).

Vedi anche: 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

Dove: r_p periapsis distance; r_a apoapsis distance

Vedi anche: 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.

Vedi anche: Solar day

Sidereal month

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

Vedi anche: 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

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

Vedi anche: 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.

Vedi anche: 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)

Vedi anche: 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

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

Vedi anche: 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.

Vedi anche: 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))

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

Vedi anche: Specific orbital energy, Circular orbital speed

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