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2. Comoving and proper distances - Wikipedia

   en.wikipedia.org/wiki/Comoving_and_proper_distances

   where a(t′) is the scale factor, t e is the time of emission of the photons detected by the observer, t is the present time, and c is the speed of light in vacuum.. Despite being an integral over time, this expression gives the correct distance that would be measured by a hypothetical tape measure at fixed time t, i.e. the "proper distance" (as defined below) after accounting for the time ...

3. Lunar distance - Wikipedia

   en.wikipedia.org/wiki/Lunar_distance

   Lunar distance (LD or ), or Earth–Moon characteristic distance, is a unit of measure in astronomy. More technically, it is the semi-major axis of the geocentric lunar orbit . The lunar distance is on average approximately 385,000 km (239,000 mi), or 1.28 light-seconds ; this is roughly 30 times Earth's diameter or 9.5 times Earth's ...

4. Astronomical unit - Wikipedia

   en.wikipedia.org/wiki/Astronomical_unit

   Distance light travels in one Julian year (365.25 days) – Oort cloud: 75,000: ± 25,000: Distance of the outer limit of Oort cloud from the Sun (estimated, corresponds to 1.2 light-years) – Parsec: 206,265 – One parsec. The parsec is defined in terms of the astronomical unit, is used to measure distances beyond the scope of the Solar ...

5. Interstellar travel - Wikipedia

   en.wikipedia.org/wiki/Interstellar_travel

   Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast difference in the scale of the involved distances. Whereas the distance between any two planets in the ...

6. Light-year - Wikipedia

   en.wikipedia.org/wiki/Light-year

   A light-year, alternatively spelled light year ( ly ), is a unit of length used to express astronomical distances and is equal to exactly 9,460,730,472,580.8 km (Scientific notation: 94,607.304725808 x 10 8 km) , which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that ...

7. Faster-than-light - Wikipedia

   en.wikipedia.org/wiki/Faster-than-light

   In the context of this article, "faster-than-light" means the transmission of information or matter faster than c, a constant equal to the speed of light in vacuum, which is 299,792,458 m/s (by definition of the metre) [3] or about 186,282.397 miles per second. This is not quite the same as traveling faster than light, since: