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  2. Light-year - Wikipedia

    en.wikipedia.org/wiki/Light-year

    Voyager 1 as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth. 10 0: 1.6 × 10 0 ly: The Oort cloud is approximately two light-years in diameter. Its inner boundary is speculated to be at 50 000 au ≈ 0.8 ly, with its outer edge at 100 000 au ≈ 1.6 ly. 2.0 × 10 0 ly

  3. Speed of light - Wikipedia

    en.wikipedia.org/wiki/Speed_of_Light

    A light-year is the distance light travels in one Julian year, around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs. In round figures, a light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri, the closest star to Earth after the Sun, is around 4.2 light-years away. [89]

  4. Unit of time - Wikipedia

    en.wikipedia.org/wiki/Unit_of_time

    In early Roman times, the interval between censuses. decade: 10 yr: indiction: 15 yr: Interval for taxation assessments (Roman Empire). gigasecond: 10 9 s: About 31.7 years. jubilee: 50 yr: century: 100 yr: millennium: 1000 yr: Also called "kiloannum". Age: 2 148 and 2/3 of a year: A superstitious unit of time used in astrology, each of them ...

  5. Distance measure - Wikipedia

    en.wikipedia.org/wiki/Distance_measure

    This distance is the time that it took light to reach the observer from the object multiplied by the speed of light. For instance, the radius of the observable universe in this distance measure becomes the age of the universe multiplied by the speed of light (1 light year/year), which turns out to be approximately 13.8 billion light years.

  6. Time dilation - Wikipedia

    en.wikipedia.org/wiki/Time_dilation

    Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them (special relativity), or a difference in gravitational potential between their locations (general relativity). When unspecified, "time dilation" usually refers to the effect due to velocity.

  7. Equation of time - Wikipedia

    en.wikipedia.org/wiki/Equation_of_time

    Its value is 0, 1, or 2 at different times of the year. Subtracting it leaves a small positive or negative fractional number of half turns, which is multiplied by 720, the number of minutes (12 hours) that the Earth takes to rotate one half turn relative to the Sun, to get the equation of time.

  8. Sidereal time - Wikipedia

    en.wikipedia.org/wiki/Sidereal_time

    Animation showing the difference between a sidereal day and a solar day. Sidereal time ("sidereal" pronounced / saɪˈdɪəriəl, sə -/ sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky.

  9. Twin paradox - Wikipedia

    en.wikipedia.org/wiki/Twin_paradox

    In their rest frame the distance between the Earth and the star system is α d = 0.6 × 4 = 2.4 light years (length contraction), for both the outward and return journeys. Each half of the journey takes α d / v = 2.4 / 0.8 = 3 years, and the round trip takes twice as long (6 years). Their calculations show that they will arrive home having ...