It makes the conversion between space and time as these are but
measurements of lengths in different directions in the same
4-dimensional geometrical space called "space-time".
As we observe astronomic objects by their light, we see them as
they were when their light was emitted, thus with a delay that we
commonly use to express their distances: 1 light-second = the
distance crossed by light in 1 second ; 1 light-year = the
distance crossed in 1 year, and so on. This can have importance :
Ole Rømer first demonstrated in 1676 that light travelled
at a finite speed by studying the apparent motion of Jupiter's
moon Io.
| Length of the Earth's equator |
40,075 km | 0.134 s |
| Geostationary orbit radius |
35 786 km | 0.119 s |
| Jupiter's mean radius |
69,911 km |
0.233 s |
| Proxima Centauri's radius (the nearest star,
a red dwarf invisible to the naked eye) |
98,000 km | 0.327 s |
| Moon's semi-major axis (mean distance to Earth's center) | 384 400 km | 1.3 s |
| Io's semi-major axis (nearest Jupiter's big
satellite) |
421,700 km |
1.41 s |
| Sun's radius | 696 342 km | 2.32 s |
| Sirius's radius |
1,191,000 km |
3.97 s |
| Callisto's semi-major axis (furthest
Jupiter's big satellite) |
1,882,700 km | 6.28 s |
| Radius of the galactic black hole |
27,000,000 km |
90 s |
| Sun-Earth |
149,598,261 km | 8min 19s |
| Antares's radius |
615,000,000 km |
34.2 min |
| Sun-Jupiter |
778,547,200 km | 43 min |
| Sun-Neptune |
4,503,443,661 km | 4h 10min |
| Average distance between neighboring stars in
the center of the Omega Centauri globular cluster |
1 month |
|
| Distance between Proxima Centauri and Alpha
Centauri |
4 months | |
| Closest approach of Gliese 710 to the Sun after 1.4 million years from now (but now at 63.8 light-years) | 1 year |
|
| Distance to Alpha Centauri (nearest visible
star) |
4.37 yr | |
| Distance to Sirius (brightest star in
appearance and second nearest visible star) |
8.6 yr |
|
| Size of Omega Centauri (the biggest globular cluster of our galaxy) | 86 yr |
|
| Width of the galactic disk |
≈1500 yr |
|
| Distance to Omega Centauri |
16,000 yr |
|
| Distance to the galactic center |
28,000 yr |
|
| Diameter of our galaxy |
100 k yr |
|
| Distance to the Large Magellanic Cloud (the
main satellite of the Milky Way) |
160 k yr |
|
| Distance between Andromeda and Triangulum
(M33) galaxies |
750 k yr |
|
| Distance to the Andromeda Galaxy (the main
member of our Local group of galaxies, followed by our Milky
Way and Triangulum) |
2 M yr | |
| Distance to NGC 55, the next significant
galaxy outside the Local Group |
7 M yr | |
| Distance to the Virgo Galaxy Cluster (the
biggest nearby galaxy cluster, at the heart of the Virgo
supercluster we belong to) |
50 M yr |
The speed of light has physical effects in the following contexts: