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NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team (CC BY 4.0)

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.

Àwọn ìṣàmúlò-ètò r_a = a (1 + e)

Wò nípa: Periapsis

Argument of periapsis (ω)

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

Àwọn ìṣàmúlò-ètò ϖ = Ω + ω

Àwọn ààyè-iṣẹ́: ϖ longitude of periapsis; Ω longitude of the ascending node

Wò nípa: 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.

Wò nípa: 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°.

Wò nípa: Declination, Solstice, Equinox

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).

Wò nípa: Magnitude, Light pollution

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.

Àwọn ìṣàmúlò-ètò v_c = √(μ / r)

Àwọn ààyè-iṣẹ́: μ gravitational parameter; r distance from the centre

Wò nípa: 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.

Wò nípa: Magnitude

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.

Àwọn ìṣàmúlò-ètò sin δ = sin ε · sin λ

Àwọn ààyè-iṣẹ́: δ Sun's declination; ε obliquity of the ecliptic; λ Sun's ecliptic longitude

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò Δv = v_e ln(m₀ / m_f)

Àwọn ààyè-iṣẹ́: v_e effective exhaust speed; m₀, m_f initial and final mass

Wò nípa: 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.

Wò nípa: Planet, Kuiper belt

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.

Wò nípa: 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).

Wò nípa: 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.

Wò nípa: 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).

Àwọn ìṣàmúlò-ètò r = a(1 − e²) / (1 + e cos ν)

Àwọn ààyè-iṣẹ́: a semi-major axis; e eccentricity; ν true anomaly

Wò nípa: 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.

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò v_esc = √(2μ / r)

Wò nípa: Circular orbital speed, Specific orbital energy

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.

Wò nípa: Orbit, Gravity

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.

Àwọn ìṣàmúlò-ètò μ = G M

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò F = G M m / r²

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

Wò nípa: Gravitational parameter (μ), Free fall

Hohmann transfer

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

Àwọn ìṣàmúlò-ètò a_t = (r₁ + r₂) / 2

Wò nípa: Delta-v, Vis-viva equation

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.

Wò nípa: Longitude of the ascending node (Ω), Retrograde

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.

Àwọn ìṣàmúlò-ètò T² / a³ = 4π² / μ

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

Wò nípa: 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.

Wò nípa: Main asteroid belt, Dwarf planet

Light pollution

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

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò t = d / c

Àwọn ààyè-iṣẹ́: d distance; c speed of light, 299,792.458 km/s

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò 1 ly = c × 365.25 × 86,400 s

Àwọn ààyè-iṣẹ́: c speed of light, 299,792.458 km/s

Wò nípa: 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.

Wò nípa: Node, Inclination (i), Argument of periapsis (ω)

Magnitude

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

Àwọn ìṣàmúlò-ètò m₁ − m₂ = −2.5 log₁₀(F₁ / F₂)

Àwọn ààyè-iṣẹ́: m magnitude; F observed flux

Wò nípa: 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.

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò M = E − e sin E

Àwọn ààyè-iṣẹ́: E eccentric anomaly; e eccentricity

Wò nípa: 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.

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò Δv = √(v∞² + v_esc²) − v_c

Àwọn ààyè-iṣẹ́: v∞ hyperbolic excess speed; v_esc escape speed at the burn; v_c circular speed at the burn

Wò nípa: 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).

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò T = 2π √(a³/μ)

Àwọn ààyè-iṣẹ́: a semi-major axis; μ gravitational parameter of the central body

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò r_p = a (1 − e)

Wò nípa: 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.

Wò nípa: 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.

Wò nípa: Dwarf planet, Orbit

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°).

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò a = (r_p + r_a) / 2

Àwọn ààyè-iṣẹ́: r_p periapsis distance; r_a apoapsis distance

Wò nípa: 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.

Wò nípa: Solar day

Sidereal month

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

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò 1/P_solar = 1/P_rot − 1/P_orb

Àwọn ààyè-iṣẹ́: P_rot sidereal rotation period (negative if retrograde); P_orb orbital period

Wò nípa: 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.

Wò nípa: Equinox, Declination

Specific orbital energy

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

Àwọn ìṣàmúlò-ètò ε = v²/2 − μ/r = −μ / (2a)

Wò nípa: 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.

Àwọn ìṣàmúlò-ètò 1/P_syn = 1/P_sid − 1/P_year

Àwọn ààyè-iṣẹ́: P_sid sidereal month, 27.322 days; P_year sidereal year, 365.256 days

Wò nípa: Sidereal month

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.

Wò nípa: Eclipse

Vis-viva equation

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

Àwọn ìṣàmúlò-ètò v = √(μ (2/r − 1/a))

Àwọn ààyè-iṣẹ́: μ gravitational parameter; r current distance; a semi-major axis

Wò nípa: Specific orbital energy, Circular orbital speed

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