Introduction Astronomy Tools Concepts 1. Electromagnetic Spectrum 2. Atmosphere Limitations 3. Space Observations Equipment 1. Telescopes 2. Radio 3. Space Tools 4. Photography 5. Spectroscopy 6. Computers 7. Advanced Methods 8. Radio Astronomy Basic Mathematics Algebra Statistics Geometry Scientific Notation Log Scales Calculus Physics Concepts - Basic Units of Measure - Mass & Density - Temperature - Velocity & Acceleration - Force, Pressure & Energy - Atoms - Quantum Physics - Nature of Light Formulas - Brightness - Cepheid Rulers - Distance - Doppler Shift - Frequency & Wavelength - Hubble's Law - Inverse Square Law - Kinetic Energy - Luminosity - Magnitudes - Convert Mass to Energy - Kepler & Newton - Orbits - Parallax - Planck's Law - Relativistic Redshift - Relativity - Schwarzschild Radius  - Synodic & Sidereal Periods - Sidereal Time - Small Angle Formula - Stellar Properties  - Stephan-Boltzmann Law - Telescope Related - Temperature - Tidal Forces - Wien's Law Constants Computer Models Additional Resources 1. Advanced Topics 2. Guest Contributions
 Physics - Formulas - Sidereal Time Astronomers used coordinate systems to find and map objects. Because Earth's time is not exact, Astronomers rely on Sidereal time.A sidereal day is two successive upper meridian crossings by the Sun of the Vernal Equinox - of course and easier way to remember this is that a sidereal day is measured by a complete Earth rotation with respect to the stars (not the Sun which would be a standard Solar day). Vernal Equinox is when the Sun passes the Celestial Equator between the northern and southern hemisphere. Vernal Equinox is also known as the first day of Spring - or (around) March 21. Midnight at the Vernal Equinox is 00:00 hours local Sidereal time. It is also important to understand that Astronomer's often use different times than we are used to - for example our watches are in sync with the Sun. This means that one day is equal to Earth's rotation with respect to the Sun. This is called a Solar day. For more accuracy, Astronomer's use Sidereal time, Earth's rotation with respect to the stars. This means that the same constellation appears at the same place after a complete rotation of the Earth. When comparing a Solar day to a Sidereal day, subtract 4 minutes from each other: Sidereal Day = Solar Day - 4 minutes This is why the night sky is different depending on the time of year, they rise 4 minutes earlier each night. So why 4 minutes? A complete circle is 360° and Earth orbits the Sun in 365.25 days, or about 1° a day. An Earth rotation is also 360 which is 24 hours (or 1440 minutes). Divide 360° into 1440 minutes and there is 4 minutes left over. To match with Sidereal time, the Earth must rotate 1° more for the Sun to appear in the same spot day by day. A Sidereal Month is also used, and is the orbit of the Moon with respect to the stars which is 27.3 days. A Synodic Month is the "standard" Moon orbit about the Earth - from full moon to full moon - which is 29.5 days. Back to Top