The one body whose position a birth date alone fixes almost perfectly.
The Sun moves about a degree a day, so a date without a time still places it to within half a degree. Only within roughly a day of a sign boundary does the birth time change which sign it falls in — the cusp cases this site flags across the catalogue.
Its speed is not constant. The Earth's orbit is an ellipse, so the Sun appears to move fastest in early January near perihelion (1.019°/day, in Capricorn) and slowest in early July near aphelion (0.953°/day, in Cancer). That is why Capricorn lasts about 29.4 days and Cancer about 31.5.
0° Aries — the March equinox — is the origin of the whole coordinate system. Every ecliptic longitude on this site is measured from the Sun's position at that moment.
What it is
The star. In a geocentric chart, the Earth's orbit seen from inside it.