What can you learn from a photon?

  1. Direction: Where did it come from?
    Gives you (X,Y) for a source, and potentially dX/dt, dY,dt, dX2/dt2, etc...  
    The measurement of astronomical positions is called"astrometry"

  2. Intensity: How many photons are there?
     Depends on both intrinsic luminosity and distance.  Measurement of intensities is"photometry"

    Astronomers frequently measure flux on a logarithmic scale called the magnitude system.

    M1-M2 = -2.5 log10(F1/F2)
    Absolute fluxes can be specified using Janskys
    1 Jy = 10-26 W m-2 Hz-4

  3. Energy (~ frequency ~ wavelength):
    The analysis of astronomical spectra provides almost all of the physical insight we have for astronomical objects. This is called "spectroscopy".
  4. Polarization
    Polarimetry can provide information about physical processes including scattering by dust, the presence of magnetic fields, and more, and can be combined with spectroscopy to get spectropolarimetry.

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