February 2025: Brilliant Venus reaches greatest brightness, shining in the western sky after sundown.
by Jeffrey L. Hunt
Brilliant Venus shines in the southwestern sky after sundown during February.
On February 1st, the crescent moon appears only 2.4° from the Evening Star. This is a spectacular sight. Do not miss it.
Venus reaches its greatest brightness on the 14th. Through a telescope the planet shows an evening crescent moon, 27% illuminated.
Greatest Brightness, Greatest Illuminated Extent Detailed
Do not confuse the greatest brightness with greatest brilliancy. For the purposes here, greatest brightness is the mid-point of a bright phase from the US Naval Observatory’s data. Greatest brilliancy is when the planet is actually brightest. Greatest brightness is February 14th while greatest brilliancy occurs February 18th, according to NASA’s JPL Horizons Ephemeris. Regardless of the name, the planet is at its brightest at midmonth and likely the human eye cannot detect the difference of 1% of the planet’s brightness from the 14th to the 18th.
Another effect is that Venus’ crescent covers the largest area of the sky. This is known as the greatest illuminated extent and it should not be confused with greatest brilliancy, which can occur up to three days on either side of the largest illuminated phase.
When we look at the moon, its globe is nearly the same size varying only about 15% during a lunar cycle from its eccentric orbit. The Full moon covers the largest area of the sky and a lighted crescent covers a smaller area.
For Venus, the planet’s distance varies greatly. The planet shows phases and Full Venus phase occurs when the planet is farthest from Earth and a crescent phase occurs when near us. On the 14th, the size of the crescent covers the largest area of the sky, and so the name greatest illuminated extent. (For those seeking a semi-technical description of this event, see this source.)
Venus rapidly drops from the evening sky during February, heading toward its inferior conjunction after midnight on March 23rd. During February, it loses 69 minutes of setting time, though setting nearly 3 hours after nightfall.
During the month, watch Venus appear lower in the sky each night, passing through greatest brightness.
Venus rapidly drops from the evening sky during February, heading toward its inferior conjunction after midnight on March 23rd. During February, it loses 69 minutes of setting time, though setting nearly 3 hours after nightfall.
During the month, watch Venus appear lower in the sky each night, passing through the brightest portion of its evening appearance.
(27 January 2025: This article was edited from Mark Gingrich’s comment on the original article and private communication, as well as additional research into the topic. His article is linked earlier in this post. Thank you for reading!)
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Greatest illuminated extent is *not* the same as greatest brilliancy. Yes, in the case of Venus, they occur close together in time — usually within five days of each other. But Mercury’s GIE often takes place two weeks before or after its peak magnitude.
If Venus acted as a perfect Lambertian reflector— that is, its surface brightness stayed absolutely constant as its phase changes — then the two events would be simultaneous. However, the clouds of Venus are not quite a perfect Lambertian surface.
This is explained in the original Journal of Association of Planetary Observers paper, published in 2001, where greatest illuminated extent was first described.
— Mark Gingrich, megingrich@gmail.com
Mark:
Thank you for the note. Sure Mercury’s brightness cycle is not related the GIE. Thanks for the reference, I’ll take a look at it.
-=-
jlh