An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



Number of Results: 5 Search : visibility
crescent Moon visibility
  دیاری ِ هلال ِ ماه   
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.

Venus visibility
  پدیداری ِ ناهید   
padidâri-ye Nâhid

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:
Day 0: Superior conjunction, "full Venus."
Day 35: Venus appears in evening sky.
Day 221: Greatest → eastern elongation, "last quarter."
Day 271: Retrogression of Venus begins.
Day 286: Disappearance from the evening sky.
Day 292: Inferior conjunction, "new Venus."
Day 298: Venus appears in morning sky.
Day 313: Retrogression ends.
Day 362: Greatest → western elongation, "first quarter."
Day 549: Disappearance from morning sky.
Day 584: Superior conjunction, "full venus."
Therefore, Venus is visible as an evening star for 286 Earth days, as a morning star for 251 days, and is invisible for 47 days.

In addition, the orbital periods of Earth and Venus are closely correlated. After 8 Earth years or 13 Venus orbits, the two planets assume almost the same relative positions -- just 0.032 percent away from a perfect orbital resonance of 8:13. After this period of about 2920 Earth days, Venus appears just 1.5° (about 22 hours) in advance of its former position. Moreover, Venus exhibit → phases because its orbit lies within the Earth's. When Venus situated on the far side of the Sun from Earth, the planet is fully illuminated from our point of view. But its disk is small, just 10'' across, because it is nearly 300 million km away. When Venus is almost closest to Earth, on the near side of the Sun, it's about 60 million km away. Then it appears as a slender but much brighter crescent with a disk nearly 50'' across. See also → transit of Venus.

Venus; → visibility.

  دیاری، پدیداری   
diyâri, padidâri (#)

Fr.: visibilité   

General: The state or fact of being visible.
1) 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).
2) Meteo.: A measure of transparency of the atmosphere. The maximum horizontal distance at which objects can be identified. → turbidity.

visible + → -ity.

visibility function
  کریای ِ دیاری، ~ پدیداری   
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.

visibility plane
  هامن ِ دیاری، ~ پدیداری   
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.