crescent Moon visibility
diyâri-ye helâl-e mâh
Fr.: visibilité du croissant lunaire
The first sighting of the → New Moon after its → conjunction with the Sun. Although the date and time of each New Moon can be computed exactly, the visibility of the lunar → crescent as a function of the → Moon's age depends upon many factors and cannot be predicted with certainty. The sighting within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the → twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies from one month to another. The visibility depends on sky conditions and the location, experience, and preparation of the observer. Ignoring atmospheric conditions, the size and brightness of the lunar crescent depend on the → elongation which in turn depends on several factors: 1) The Moon's elongation at New Moon (the elongation of the Moon at New Moon is not necessarily 0). 2) The speed of the Moon in its elliptical orbit. 3) The distance of the Moon, and 4) The observer's location (parallax). The combined effect of the first three factors gives geocentric elongation of the Moon from the Sun at an age of one day which can vary between about 10 and 15 degrees. This large range of possible elongations in the one-day-old Moon is critical (US Naval Observatory).
→ crescent; → moon; → visibility.
padidâri-ye fariz (#)
Fr.: visibilité des franges
Optics: If the intensity in an interference fringe pattern has the maximum and minimum values Imax and Imin, the visibility is defined by the relation ν = (Imax - Imin) / (Imax + Imin), where 0 ≤ ν ≤ 1. In terms of the intensities of the two interfering waves: ν = 2(I1 . I2)1/2 / (I1 + I2).
→ fringe; → visibility
Fr.: visibilité de Vénus
The conditions under which Venus can be seen from Earth as
it travels in its orbit around the Sun.
The → synodic period of Venus, that is the time Venus takes to
be seen again from the
Earth in the same position with respect to the Sun, is 583,92 days
or just over 19 months. When Venus is between Earth and Sun
(→ inferior conjunction) or on the far side of the sun
(→ superior conjunction), it is invisible in the Sun's glare. Since its
→ greatest elongation
from the Sun is never more than 47°, Venus appears only as
"the morning star" and "the evening star."
So at its greatest → western elongation
Venus will rise about
three hours ahead of the Sun and at its greatest → eastern elongation
it will set about three hours after sunset.
Its entire cycle is as follows:
→ Venus; → visibility.
diyâri, padidâri (#)
1) General: The state or fact of being visible.
→ crescent Moon visibility,
→ Venus visibility.
karyâ-ye diyâri, ~ padidâri
Fr.: fonction de visibilité
The → Fourier transform of a source's → brightness distribution, weighted by the characteristics of the → interferometer's antennas.
→ visibility; → function.
hâmon-e diyâri, ~ padidâri
Fr.: plan de visibilité
In → interferometry, the projection of a → baseline onto the plane normal to the source direction defining a vector in (u,v) space, measured in wavelength units.
→ visibility; → plane.