پدیداری ِ ناهید
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.