Star Sizes "Narrabri Observatory in Australia has a strange-looking instrument that is proving to be very successful for measuring the angular diameters
of bright stars.
Thirty arc minutes apiece: It's a long-odds cosmic coincidence that the angular diameters
of the sun and moon are nearly identical - about 30 arc minutes.
This led the medieval Islamic astronomers of the early past millennium to appeal to other, pre-existing, astronomical traditions, both to justify the phenomenon and to gain a theoretical grounding for calculating the solar and lunar angular diameters
. The Indian astronomical hypotheses defined the luminaries' angular diameters
as a single variable function of their instantaneous velocity (i.e., the difference in longitude between two successive moments, e.g., two days or hours) and provided a suitable context in which an annular eclipse could be justified.
Even the best star catalogs and asteroid orbits have uncertainties that are significant compared with most asteroids' tiny angular diameters
. In the worst cases, the uncertainty may be several times the path width.
Even stellar occultations will not cause noticeable dimming because the angular diameters
of most comets at such vast distances are small compared to the angular diameters
of the stars they block.
A team led by one of us (Berger) used the CHARA Array to resolve the angular diameters
of six red dwarfs, which together with parallax measurements yield physical diameters accurate to better than 5%.
Antares itself displays one of the largest angular diameters
of any star in the sky: about 40 milliarcseconds at visible wavelengths.
New close double stars (including spectroscopic binaries) can also be resolved this way, and the angular diameters
of some stars can be measured.
Stellar interferometry is already contributing in fundamental ways to astrophysics, primarily by measuring the angular diameters
of stars and the orbits of close binary stars.
Ephemerides typically include angular diameters
, which allow comparisons among planets' apparent sizes (see the table on page 97).
Although the Almanac features a diary of monthly celestial events, it reigns supreme in providing near-daily tabulated positions of celestial bodies - called ephemerides - including physical values like planetary angular diameters
, visual magnitudes, and distances from Earth and the Sun.