An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The average time interval between two successive → upper transits of the → mean equinox.

→ mean; → sidereal; → day.

The average length of the apparent solar day. In other words, the interval between successive transits of the mean Sun for a given observer.

→ mean; → solar; → day.

The middle of the day; noon or the time centering around noon.

→ mid-; → day.

The time required for the plane of a freely suspended → Foucault pendulum to complete an apparent rotation about the local vertical. It is given by T = 23.9344 / sin φ in hours, where φ represents the latitude of the place. For Paris it is 31h 47m 38s; for the poles it 23.9344 h and for the equator it is ∞ since the plane of pendulum does not turn.

→ pendulum; → day.

In polar regions, the portion of the year when the Sun is continuously in the sky. Its length changes from twenty hours at the Arctic/Antarctic Circle (latitude 66°33' N or S) to 186 days at the North/South Pole.

→ polar; → day.

Same as → present epoch, → today, → current cosmological epoch.

→ present; → day.

The present number of stars on the → main sequence per unit logarithmic mass interval per square parsec. The PDMF is the basis for deriving the → initial mass function (IMF). This mass function is not corrected for stellar evolution nor losses through stellar deaths.

→ present; → day; → mass; → function.

The period of → Earth's rotation around its axis, the mean value of which is about 23h 56m 04.092s (23.934 470 hours). In other words, the interval between two successive passages of a star across a given → meridian. The sidereal day is 3 minutes 56 seconds shorter than the → mean solar day. The reason is that Earth moves a little less than a degree around the Sun during the time it takes for one full axial rotation. So, for the Sun to appear on the same meridian in the sky again after one full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is also why the stars rise and set about 4 min earlier each day.

→ sidereal; → day.

The length of time between two successive transits of the Sun over the same meridian.

→ solar; → day.

1) The present day, time, or age.
2) The present epoch in the cosmic evolution since the → Big Bang characterizd by the → redshift  z = 0.

M.E. today, from O.E. todæge, to dæge "on (this) day," from to "at, on" + dæge, dative of dæg "→ day."

Emruz "today," from Mid.Pers. imrôc, imrôz, from im "this; here" + rôz, ruz, → day.