question: Bombay Time is set at 4 hours and 51 minutes ahead of the mean solar time at what observatory?
answer: Royal Observatory
context: Title: Bombay Time
  Passage: Bombay Time was one of the two official time zones established in British India in 1884.  The time zone was established during the International Meridian Conference held at Washington, D.C in the United States in 1884.  It was then decided that India would have two time zones, with Calcutta (now Kolkata) using the 90th meridian east and Bombay (now Mumbai) the 75th meridian east.  Bombay Time was set at 4 hours and 51 minutes ahead of Greenwich Mean Time (GMT).
  Title: Atlantic Time Zone
  Passage: The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC-4; during part of the year some parts of it observe daylight saving time by instead subtracting only three hours (UTC-3).  The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
  Title: Universal Time
  Passage: Universal Time (UT) is a time standard based on Earth's rotation.  It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK.  In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see below).  All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time.  UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.
  Title: Greenwich Mean Time
  Passage: Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London.  GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC).  Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalised) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes.
  Title: Newfoundland Time Zone
  Passage: The Newfoundland Time Zone is a geographic region that keeps time by subtracting 3 ⁄ hours from Coordinated Universal Time (UTC) during standard time, resulting in ; or subtracting 2 ⁄ hours during daylight saving time.  The clock time in this zone is based on the mean solar time of the meridian 52 degrees and 30 arcminutes west of the Greenwich Observatory.
  Title: Equation of time
  Passage: The equation of time describes the discrepancy between two kinds of solar time.  The word "equation" is used in the medieval sense of "reconcile a difference".  The two times that differ are the "apparent" solar time, which directly tracks the diurnal motion of the Sun, and "mean" solar time, which tracks a theoretical mean Sun with noons 24 hours apart.  Apparent solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial.  "Mean" solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would resolve to zero.
citations: Greenwich Mean Time Bombay Time