# Space & Astronomy Almanac

## Solar System Overview

| Property | Value |
|----------|-------|
|  Age  | ~4.6 billion years |
|  Location  | Orion Arm of Milky Way, ~26,000 light-years from galactic center |
|  Number of planets  | 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) |
|  Number of dwarf planets  | 5 recognized (Pluto, Eris, Makemake, Haumea, Ceres) + many candidates |
|  Kuiper Belt  | Starts ~30 AU, extends to ~50 AU |
|  Oort Cloud  | Theoretical shell from ~2,000 to ~100,000 AU |

---

## The Sun

| Property | Value | Notes |
|----------|-------|-------|
|  Type  | G2V (yellow dwarf) | Main sequence star |
|  Mass  | 1.989 × 10³⁰ kg | 99.86% of solar system mass |
|  Diameter  | 1,392,700 km | 109× Earth's diameter |
|  Surface temperature  | ~5,500°C (5,778 K) | Photosphere temperature |
|  Core temperature  | ~15 million °C | Where fusion occurs |
|  Age  | ~4.6 billion years | About halfway through life |
|  Lifespan remaining  | ~5 billion years | Will become red giant |
|  Luminosity  | 3.828 × 10²⁶ watts | Total energy output |
|  Rotation period  | 25 days (equator), 35 days (poles) | Differential rotation |
|  Composition  | 73% H, 25% He, 2% other | By mass |
|  Distance from Earth  | 149.6 million km (1 AU) | Average; varies with orbit |

---

## Planets - Basic Data

| Planet | Distance from Sun (AU) | Distance from Sun (million km) | Orbital Period | Day Length |
|--------|------------------------|--------------------------------|----------------|------------|
|  Mercury  | 0.39 | 57.9 | 88 days | 59 days |
|  Venus  | 0.72 | 108.2 | 225 days | 243 days (retrograde) |
|  Earth  | 1.00 | 149.6 | 365.25 days | 24 hours |
|  Mars  | 1.52 | 227.9 | 687 days | 24.6 hours |
|  Jupiter  | 5.20 | 778.5 | 11.9 years | 9.9 hours |
|  Saturn  | 9.54 | 1,427 | 29.5 years | 10.7 hours |
|  Uranus  | 19.19 | 2,871 | 84 years | 17.2 hours (retrograde) |
|  Neptune  | 30.07 | 4,498 | 165 years | 16.1 hours |

---

## Planets - Orbital Data

| Planet | Orbital Velocity (km/s) | Orbital Eccentricity | Inclination to Ecliptic |
|--------|------------------------|---------------------|------------------------|
|  Mercury  | 47.9 | 0.206 (most eccentric) | 7.0° |
|  Venus  | 35.0 | 0.007 | 3.4° |
|  Earth  | 29.8 | 0.017 | 0° (reference) |
|  Mars  | 24.1 | 0.093 | 1.9° |
|  Jupiter  | 13.1 | 0.048 | 1.3° |
|  Saturn  | 9.7 | 0.056 | 2.5° |
|  Uranus  | 6.8 | 0.046 | 0.8° |
|  Neptune  | 5.4 | 0.010 | 1.8° |

---

## Planets - Physical Characteristics

| Planet | Diameter (km) | Diameter (×Earth) | Mass (×Earth) | Gravity (×Earth) | Moons | Rings |
|--------|---------------|-------------------|---------------|------------------|-------|-------|
|  Mercury  | 4,879 | 0.38 | 0.055 | 0.38 | 0 | No |
|  Venus  | 12,104 | 0.95 | 0.815 | 0.91 | 0 | No |
|  Earth  | 12,742 | 1.00 | 1.000 | 1.00 | 1 | No |
|  Mars  | 6,779 | 0.53 | 0.107 | 0.38 | 2 | No |
|  Jupiter  | 139,820 | 10.97 | 317.8 | 2.53 | 95+ | Yes (faint) |
|  Saturn  | 116,460 | 9.14 | 95.2 | 1.07 | 146+ | Yes (prominent) |
|  Uranus  | 50,724 | 3.98 | 14.5 | 0.89 | 27+ | Yes (faint) |
|  Neptune  | 49,244 | 3.86 | 17.1 | 1.14 | 14+ | Yes (faint) |

---

## Earth's Moon

| Property | Value | Notes |
|----------|-------|-------|
|  Distance from Earth  | 384,400 km | Average; varies 356,500-406,700 km |
|  Diameter  | 3,474 km | About 1/4 Earth's diameter |
|  Mass  | 7.342 × 10²² kg | 1/81 of Earth's mass |
|  Orbital period  | 27.3 days | Sidereal month |
|  Rotation period  | 27.3 days | Tidally locked (same side faces Earth) |
|  Lunar cycle  | 29.5 days | Synodic month (new moon to new moon) |
|  Surface gravity  | 1.62 m/s² | 1/6 of Earth's |
|  Surface temperature  | -173°C to 127°C | Extreme variation (no atmosphere) |
|  Age  | ~4.5 billion years | Formed from giant impact |
|  Atmosphere  | Virtually none | Trace exosphere |

---

## Major Moons of Other Planets

| Moon | Planet | Diameter (km) | Notable Features |
|------|--------|---------------|------------------|
|  Io  | Jupiter | 3,643 | Most volcanically active body; sulfur surface |
|  Europa  | Jupiter | 3,122 | Subsurface ocean; smooth ice surface |
|  Ganymede  | Jupiter | 5,268 | Largest moon in solar system; own magnetic field |
|  Callisto  | Jupiter | 4,821 | Heavily cratered; ancient surface |
|  Titan  | Saturn | 5,150 | Dense atmosphere; liquid methane lakes |
|  Enceladus  | Saturn | 504 | Ice geysers; subsurface ocean |
|  Mimas  | Saturn | 396 | "Death Star" crater; Herschel crater |
|  Triton  | Neptune | 2,707 | Retrograde orbit; nitrogen geysers; captured object |
|  Miranda  | Uranus | 472 | Extreme geological features; "Frankenstein moon" |
|  Charon  | Pluto | 1,212 | Half Pluto's diameter; double planet system |

---

## Dwarf Planets

| Name | Location | Distance from Sun (AU) | Diameter (km) | Orbital Period |
|------|----------|----------------------|---------------|----------------|
|  Ceres  | Asteroid Belt | 2.77 | 946 | 4.6 years |
|  Pluto  | Kuiper Belt | 39.5 | 2,377 | 248 years |
|  Eris  | Scattered Disc | 68 | 2,326 | 557 years |
|  Makemake  | Kuiper Belt | 45.8 | 1,430 | 305 years |
|  Haumea  | Kuiper Belt | 43.2 | 1,632 (elongated) | 284 years |

---

## Asteroid Belt

| Property | Value |
|----------|-------|
|  Location  | Between Mars and Jupiter, 2.2-3.2 AU |
|  Number of asteroids  | Millions (hundreds of thousands catalogued) |
|  Total mass  | ~3% of Moon's mass |
|  Largest asteroid  | Ceres (946 km diameter, now classified as dwarf planet) |
|  Other large asteroids  | Vesta (525 km), Pallas (512 km), Hygiea (434 km) |

---

## Distance Units

| Unit | Equivalent | Usage |
|------|-----------|--------|
|  Kilometer (km)  | 1,000 meters | Planetary distances, moon distances |
|  Astronomical Unit (AU)  | 149,597,871 km | Solar system distances |
|  Light-second  | 299,792 km | Earth-Moon ~1.3 light-seconds |
|  Light-minute  | 17,987,547 km | Sun-Earth ~8.3 light-minutes |
|  Light-year (ly)  | 9.461 trillion km | Interstellar distances |
|  Parsec (pc)  | 3.26 light-years | Astronomical measurements |
|  Kiloparsec (kpc)  | 3,260 light-years | Galactic distances |
|  Megaparsec (Mpc)  | 3.26 million light-years | Intergalactic distances |

---

## Brightest Stars

| Star | Constellation | Apparent Magnitude | Distance (ly) | Type | Notes |
|------|---------------|-------------------|---------------|------|-------|
|  Sirius  | Canis Major | -1.46 | 8.6 | A1V | Brightest star in night sky; "Dog Star" |
|  Canopus  | Carina | -0.74 | 310 | A9II | Second brightest; southern hemisphere |
|  Arcturus  | Boötes | -0.05 | 37 | K0III | Orange giant; "follow the arc" from Big Dipper |
|  Alpha Centauri A  | Centaurus | -0.01 | 4.37 | G2V | Nearest star system to Sun |
|  Vega  | Lyra | 0.03 | 25 | A0V | Summer Triangle; former pole star |
|  Capella  | Auriga | 0.08 | 43 | G8III | Actually a quadruple star system |
|  Rigel  | Orion | 0.13 | 860 | B8Ia | Blue supergiant; Orion's foot |
|  Procyon  | Canis Minor | 0.38 | 11.5 | F5IV | "Little Dog Star"; Winter Triangle |
|  Betelgeuse  | Orion | 0.42 (variable) | 650 | M1-2Ia | Red supergiant; may go supernova "soon" |
|  Altair  | Aquila | 0.77 | 17 | A7V | Summer Triangle; rapidly rotating |

---

## Nearest Stars

| Star/System | Distance (ly) | Type | Notes |
|-------------|---------------|------|-------|
|  Proxima Centauri  | 4.24 | M5.5Ve | Nearest star; part of Alpha Centauri system |
|  Alpha Centauri A & B  | 4.37 | G2V & K1V | Binary system; A similar to Sun |
|  Barnard's Star  | 5.96 | M4Ve | Fast proper motion; flare star |
|  Wolf 359  | 7.86 | M6V | Very dim red dwarf |
|  Lalande 21185  | 8.29 | M2V | Red dwarf with planetary system |
|  Sirius A & B  | 8.60 | A1V & DA | Brightest star; white dwarf companion |
|  Luyten 726-8 A & B  | 8.73 | M5.5Ve & M6Ve | Binary flare stars |
|  Ross 154  | 9.69 | M3.5Ve | Flare star |
|  Ross 248  | 10.32 | M5.5Ve | Dim red dwarf |
|  Epsilon Eridani  | 10.52 | K2V | Planetary system; young star |

---

## Notable Star Systems

| System | Distance (ly) | Type | Notable Features |
|--------|---------------|------|------------------|
|  Polaris  | 433 | F7Ib | North Star; triple system |
|  Betelgeuse  | 650 | M1-2Ia | Red supergiant; variable; may explode soon |
|  Antares  | 550 | M1.5Iab | Red supergiant; "rival of Mars" |
|  Aldebaran  | 65 | K5III | Orange giant; "eye of the bull" in Taurus |
|  Deneb  | 2,600 | A2Ia | White supergiant; Summer Triangle |
|  Regulus  | 79 | B8IVn | "Heart of the Lion"; rapidly rotating |
|  Spica  | 250 | B1III-IV | Binary system; "ear of wheat" in Virgo |
|  Fomalhaut  | 25 | A3V | Debris disk; exoplanet |
|  Pollux  | 34 | K0III | Orange giant; exoplanet; Gemini twin |
|  Castor  | 52 | A1V/A2Vm | Sextuple star system; Gemini twin |

---

## Constellations - Zodiac

| Constellation | Abbreviation | Brightest Star | Best Visibility | Mythological Figure |
|---------------|--------------|----------------|-----------------|---------------------|
|  Aries  | Ari | Hamal | Nov-Dec | Ram |
|  Taurus  | Tau | Aldebaran | Dec-Jan | Bull |
|  Gemini  | Gem | Pollux | Jan-Feb | Twins |
|  Cancer  | Cnc | Tarf (β Cnc) | Feb-Mar | Crab |
|  Leo  | Leo | Regulus | Mar-Apr | Lion |
|  Virgo  | Vir | Spica | Apr-May | Maiden |
|  Libra  | Lib | Zubeneschamali | May-Jun | Scales |
|  Scorpius  | Sco | Antares | Jun-Jul | Scorpion |
|  Sagittarius  | Sgr | Kaus Australis | Jul-Aug | Archer |
|  Capricornus  | Cap | Deneb Algedi | Aug-Sep | Sea-goat |
|  Aquarius  | Aqr | Sadalsuud | Sep-Oct | Water-bearer |
|  Pisces  | Psc | Alpherg | Oct-Nov | Fish |

---

## Constellations - Major Northern

| Constellation | Abbreviation | Brightest Star | Key Features | How to Find |
|---------------|--------------|----------------|--------------|-------------|
|  Ursa Major  | UMa | Alioth | Big Dipper asterism; circumpolar | Most recognizable northern pattern |
|  Ursa Minor  | UMi | Polaris | Little Dipper; contains North Star | End of Big Dipper points to Polaris |
|  Cassiopeia  | Cas | Schedar | W or M shape; circumpolar | Opposite Big Dipper across Polaris |
|  Cygnus  | Cyg | Deneb | Northern Cross; Summer Triangle | Flying down the Milky Way in summer |
|  Lyra  | Lyr | Vega | Small but bright; Summer Triangle | Bright Vega easy to spot |
|  Aquila  | Aql | Altair | Spread wings; Summer Triangle | Between Cygnus and Sagittarius |
|  Orion  | Ori | Rigel | Belt and sword; The Hunter | Most recognizable winter constellation |
|  Boötes  | Boo | Arcturus | Kite shape; The Herdsman | Follow arc of Big Dipper handle |
|  Auriga  | Aur | Capella | Pentagon shape; The Charioteer | Near Orion and Taurus |
|  Perseus  | Per | Mirfak | Contains Algol; The Hero | Between Cassiopeia and Pleiades |

---

## Constellations - Major Southern

| Constellation | Abbreviation | Brightest Star | Key Features | Visibility |
|---------------|--------------|----------------|--------------|------------|
|  Crux  | Cru | Acrux | Southern Cross; smallest constellation | Deep southern hemisphere; points to south pole |
|  Centaurus  | Cen | Alpha Centauri | Contains nearest star system | Best in southern hemisphere |
|  Carina  | Car | Canopus | Part of ancient Argo Navis; second brightest star | Southern hemisphere |
|  Scorpius  | Sco | Antares | Heart of scorpion; distinctive shape | Visible both hemispheres, better south |
|  Sagittarius  | Sgr | Kaus Australis | Teapot asterism; Milky Way center | Summer, both hemispheres |
|  Eridanus  | Eri | Achernar | Long winding river | Mostly southern |
|  Hydra  | Hya | Alphard | Longest constellation | Crosses equator |
|  Pavo  | Pav | Peacock | The Peacock | Southern hemisphere |
|  Grus  | Gru | Alnair | The Crane | Southern hemisphere |
|  Tucana  | Tuc | α Tucanae | Contains Small Magellanic Cloud | Southern hemisphere |

---

## Celestial Navigation Basics

| Concept | Description | Practical Use |
|---------|-------------|---------------|
|  Celestial Equator  | Projection of Earth's equator onto sky | Divides northern and southern sky |
|  Ecliptic  | Sun's apparent path through sky | Zodiac constellations lie along it |
|  Right Ascension (RA)  | Celestial longitude (0-24 hours) | Locates objects east-west |
|  Declination (Dec)  | Celestial latitude (-90° to +90°) | Locates objects north-south |
|  Zenith  | Point directly overhead | Altitude = 90° |
|  Meridian  | Line from north to south through zenith | Objects cross at highest altitude |
|  Altitude  | Angle above horizon (0-90°) | Measured with sextant |
|  Azimuth  | Compass bearing (0-360°) | Direction along horizon |
|  Polaris altitude  | Equals observer's latitude | Navigation in Northern Hemisphere |
|  Culmination  | Object crosses meridian | Highest point in sky |

 Finding North: 
- Northern Hemisphere: Polaris (end of Little Dipper handle)
- Southern Hemisphere: Use Southern Cross (extend long axis ~4.5× length)

 Finding Latitude: 
- Measure altitude of Polaris = your latitude (Northern Hemisphere)
- Measure altitude of south celestial pole = your latitude (Southern Hemisphere)

---

## Messier Objects (Selected)

| Number | Name | Type | Constellation | Distance (ly) | Magnitude | Notes |
|--------|------|------|---------------|---------------|-----------|-------|
|  M1  | Crab Nebula | Supernova remnant | Taurus | 6,500 | 8.4 | 1054 AD supernova |
|  M13  | Hercules Cluster | Globular cluster | Hercules | 25,000 | 5.8 | Best northern globular |
|  M31  | Andromeda Galaxy | Spiral galaxy | Andromeda | 2.5 million | 3.4 | Nearest major galaxy |
|  M42  | Orion Nebula | Emission nebula | Orion | 1,344 | 4.0 | Visible to naked eye |
|  M44  | Beehive Cluster | Open cluster | Cancer | 610 | 3.7 | Praesepe |
|  M45  | Pleiades | Open cluster | Taurus | 444 | 1.6 | Seven Sisters |
|  M51  | Whirlpool Galaxy | Spiral galaxy | Canes Venatici | 23 million | 8.4 | Face-on spiral |
|  M57  | Ring Nebula | Planetary nebula | Lyra | 2,300 | 8.8 | Classic planetary nebula |
|  M81  | Bode's Galaxy | Spiral galaxy | Ursa Major | 12 million | 6.9 | Pairs with M82 |
|  M87  | Virgo A | Elliptical galaxy | Virgo | 53 million | 8.6 | First black hole image |

---

## Star Types & Classification

### Spectral Types (Hottest to Coolest)

| Type | Color | Temperature (K) | Example Stars | Mnemonic |
|------|-------|-----------------|---------------|----------|
|  O  | Blue | 30,000-50,000 | Zeta Ophiuchi, Alnitak |  O h |
|  B  | Blue-white | 10,000-30,000 | Rigel, Spica, Sirius |  B e |
|  A  | White | 7,500-10,000 | Vega, Altair, Sirius |  A  |
|  F  | Yellow-white | 6,000-7,500 | Procyon, Polaris |  F ine |
|  G  | Yellow | 5,200-6,000 | Sun, Alpha Centauri A |  G irl/Guy |
|  K  | Orange | 3,700-5,200 | Arcturus, Aldebaran |  K iss |
|  M  | Red | 2,400-3,700 | Betelgeuse, Antares, Proxima |  M e |

### Luminosity Classes

| Class | Type | Description |
|-------|------|-------------|
|  Ia  | Bright supergiant | Deneb, Rigel |
|  Ib  | Supergiant | Betelgeuse, Antares |
|  II  | Bright giant | Polaris |
|  III  | Giant | Arcturus, Aldebaran |
|  IV  | Subgiant | Procyon |
|  V  | Main sequence | Sun, Sirius, Alpha Centauri A |
|  VI  | Subdwarf | Rare, metal-poor stars |
|  VII  | White dwarf | Sirius B, Procyon B |

 Example:  Sun = G2V (G-type, main sequence)

---

## Cosmic Distance Ladder

| Method | Range | Accuracy | Application |
|--------|-------|----------|-------------|
|  Radar ranging  | Within solar system | Very high | Planet distances |
|  Parallax  | Up to ~30,000 ly | High for nearby | Stars within Milky Way |
|  Main sequence fitting  | Up to ~1 million ly | Moderate | Star clusters |
|  Cepheid variables  | Up to ~100 million ly | Good | Nearby galaxies |
|  Type Ia supernovae  | Billions of ly | Good | Distant galaxies |
|  Redshift  | Entire observable universe | Model-dependent | Cosmological distances |

 Parallax formula:  Distance (parsecs) = 1 / parallax angle (arcseconds)

---

## Time & Calendars

| Concept | Duration | Notes |
|---------|----------|-------|
|  Solar day  | 24 hours | Time between successive solar noons |
|  Sidereal day  | 23h 56m 4s | Earth's rotation relative to stars |
|  Solar year  | 365.24219 days | Time for Earth to orbit Sun |
|  Sidereal year  | 365.25636 days | Relative to fixed stars |
|  Lunar month (synodic)  | 29.53059 days | New moon to new moon |
|  Lunar month (sidereal)  | 27.32166 days | Moon's orbit relative to stars |
|  Precession cycle  | 25,772 years | Wobble of Earth's axis |
|  Julian calendar  | 365.25 days/year | Used until 1582, now 13 days off |
|  Gregorian calendar  | 365.2425 days/year | Current standard (1582-present) |
|  Leap year rule  | Every 4 years, except centuries unless divisible by 400 | 2000 was leap year, 1900 wasn't |

 Why we have leap years:  Earth's year is ~365.25 days, so we add one day every 4 years to compensate.

---

## Eclipses

### Solar Eclipses

| Type | Description | Frequency | Duration |
|------|-------------|-----------|----------|
|  Total  | Moon completely covers Sun | ~1.5 years globally | Up to 7.5 minutes |
|  Annular  | Moon doesn't quite cover Sun; "ring of fire" | ~1.5 years globally | Up to 12 minutes |
|  Partial  | Moon partially covers Sun | Common | Varies |
|  Hybrid  | Appears total in some locations, annular in others | Rare | Varies |

 Saros cycle:  Eclipses repeat every 18 years, 11 days, 8 hours

### Lunar Eclipses

| Type | Description | Frequency | Duration |
|------|-------------|-----------|----------|
|  Total  | Moon passes through Earth's umbra | ~1 year globally | Up to 107 minutes |
|  Partial  | Moon partially enters umbra | Common | Varies |
|  Penumbral  | Moon only enters penumbra (subtle) | Very common | Varies |

 Blood Moon:  During total lunar eclipse, Moon appears red due to Earth's atmosphere refracting sunlight

---

## Meteor Showers

| Name | Peak Date | Parent Body | ZHR* | Radiant Constellation |
|------|-----------|-------------|------|----------------------|
|  Quadrantids  | Jan 3-4 | Asteroid 2003 EH1 | 120 | Boötes |
|  Lyrids  | Apr 22-23 | Comet Thatcher | 18 | Lyra |
|  Eta Aquariids  | May 5-6 | Halley's Comet | 60 | Aquarius |
|  Perseids  | Aug 12-13 | Comet Swift-Tuttle | 100 | Perseus |
|  Orionids  | Oct 21-22 | Halley's Comet | 20 | Orion |
|  Leonids  | Nov 17-18 | Comet Tempel-Tuttle | 15 (storms every 33 yrs) | Leo |
|  Geminids  | Dec 13-14 | Asteroid 3200 Phaethon | 120 | Gemini |

*ZHR = Zenith Hourly Rate (ideal conditions)

 Best viewing:  After midnight, away from city lights, during new moon

---

## Space Exploration Milestones

| Year | Event | Details |
|------|-------|---------|
|  1957  | Sputnik 1 | First artificial satellite (USSR) |
|  1961  | Yuri Gagarin | First human in space (USSR) |
|  1969  | Apollo 11 | First humans on Moon: Neil Armstrong & Buzz Aldrin (USA) |
|  1971  | Salyut 1 | First space station (USSR) |
|  1977  | Voyager 1 & 2 launched | Now in interstellar space |
|  1981  | Space Shuttle | First reusable spacecraft (USA) |
|  1990  | Hubble Space Telescope | Revolutionized astronomy |
|  1998  | ISS construction begins | International Space Station |
|  2004  | Spirit & Opportunity | Mars rovers (USA) |
|  2012  | Curiosity | Mars Science Laboratory (USA) |
|  2015  | New Horizons | First Pluto flyby (USA) |
|  2019  | First black hole image | M87* by Event Horizon Telescope |
|  2021  | James Webb Space Telescope | Launched Dec 25; most powerful telescope |

---

## Useful Constants & Formulas

### Constants

| Constant | Symbol | Value |
|----------|--------|-------|
|  Speed of light  | c | 299,792,458 m/s |
|  Gravitational constant  | G | 6.674 × 10⁻¹¹ m³/(kg·s²) |
|  Planck constant  | h | 6.626 × 10⁻³⁴ J·s |
|  Solar mass  | M☉ | 1.989 × 10³⁰ kg |
|  Earth mass  | M⊕ | 5.972 × 10²⁴ kg |
|  Astronomical Unit  | AU | 1.496 × 10⁸ km |
|  Parsec  | pc | 3.086 × 10¹³ km |
|  Light-year  | ly | 9.461 × 10¹² km |

### Formulas

| Formula | Description | Variables |
|---------|-------------|-----------|
|  d = 1/p  | Parallax distance | d in parsecs, p in arcseconds |
|  m - M = 5 log(d) - 5  | Distance modulus | m = apparent mag, M = absolute mag, d in parsecs |
|  F = GMm/r²  | Gravity | G = constant, M & m = masses, r = distance |
|  v = √(GM/r)  | Orbital velocity | G = constant, M = central mass, r = orbital radius |
|  P² = a³  | Kepler's 3rd law | P in years, a in AU (for solar system) |
|  E = mc²  | Mass-energy | E = energy, m = mass, c = speed of light |
|  z = Δλ/λ  | Redshift | z = redshift, Δλ = wavelength change, λ = rest wavelength |
|  v = H₀ × d  | Hubble's Law | v = recession velocity, H₀ ≈ 70 km/s/Mpc, d = distance |

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