With luck and clear skies, you can spot Venus crossing between the Earth and the Sun this weekend.
Up for a challenge? If skies are clear, you may be able to complete a rare feat of visual athletics this coming weekend, and follow Venus on its trek from the evening and into morning sky.
Following Venus
Brilliant Venus has dominated the evening sky in early 2025 as the brightest member of the planetary parade. Now, Venus is taking the plunge as it heads towards inferior conjunction between the Earth and the Sun on Sunday, March 23rd. The exact hour of inferior conjunction occurs at 1:00 Universal Time. Venus passes just under nine degrees north of the Sun in 2025, marking a rare, once a decade optimal chance to follow the slender crescent right though its transition from the dusk and into the dawn sky.
Venus is really on the move… here’s a great study of the changing phases of the perpetually cloud-shrouded world from September 24th to March 9th, courtesy of Efrain Morales:
The changing phases of Venus, from September 2024 to March 2025. Credit: Efrain Morales.
It’s amazing that amateur observers can now actually tease out cloud detail on Venus, courtesy of newer ultraviolet filters. Venus is slimming down as it heads towards inferior conjunction between the Earth and the Sun on March 23rd… can you follow it right up to a day or two prior, or even the day of?
We once braved the Arctic chill to catch Venus on the day of inferior conjunction on January 16th, 1998 as seen from the Chena Flood Channel outside of North Pole, Alaska… it was a tough but not impossible catch, low in the ice fog.
Looking westward at dusk this week, from latitude 35 degrees north. Credit: Stellarium.
Shining at just over magnitude -4, Venus can be visible to the naked eye in the daytime near greatest elongation from the Sun… if you know exactly where to look for it. Do not attempt to spot Venus when it’s near the Sun however, as serious eye damage could result. Leading up to inferior conjunction, your best bet is to watch for Venus low to the horizon 10 minutes after sunset leading up to inferior conjunction, and 10 minutes before local sunset after. This puts the Sun safely out of view, below the horizon.
Looking east this week at dawn, from latitude 35 degrees north. Credit: Stellarium.
The relatively high angle of the ecliptic also favors northern hemisphere observers in March, as we just came off the vernal Spring equinox on March 20th. Binoculars will definitely help in your quest. Also, it’s worth noting the position of Venus versus local landmarks on the horizon from one night to the next. This act follows in the tradition of ancient astronomers of yore, only you’ll have cell towers and modern buildings for way-points, versus hilltops and stone markers.
A crescent Venus versus a modern cell tower. Credit: Dave Dickinson.
At the eyepiece of a telescope—or even in binoculars—Venus now shows prominent ‘horns’ mimicking the crescent Moon. Near inferior conjunction, these delicate features can almost seem to extend all the way around the edge of the nighttime side of the planet now facing Earthward. Can you see the horns with the naked eye? At a maximum apparent size of just under an arcminute (59.5”) across at its closest approach, Venus is right on the edge of what’s possible in terms of resolution versus the human eye.
The 8-Year Cycle of Venus
The orbit of Venus is inclined 3.4 degrees versus the ecliptic. If it were edge-on, we’d see the planet transit the Sun every pass at inferior conjunction. Venus completes 13 orbits around the Sun, for every 8 orbits of the Earth. This 8-year cycle for the planet was well known to the ancient Babylonians and the Mayans, who incorporated it into their calendars. For sky-watchers, this means that the celestial path of Venus very nearly repeats every years. But the passages at inferior conjunction for Venus versus the Sun also alternate from north to south. The 2021 and 2029 passages favor the southern hemisphere, and the 2017, and 2033 favor the northern, all in March. That makes this weekend’s passage the most favorable one for northern hemisphere observers for this decade.
What’s more, transits of the Sun for Venus are even more infrequent, occurring in pairs spaced eight years apart, once every alternating period of 121.5 and 105.5 years. The last pair happened in 2004 and 2012, and the next two won’t grace our skies until (mark your calendars) 2117 and 2125.
Venus transits the Sun, as seen by NASA’s Solar Dynamics Observatory. Credit: NASA/SDO.
Transit observations were once on the very cutting edge on astronomical science in the 18th and 19th century, as astronomers realized that precise measurements could help them pin down the value of the Earth-Sun distance. This, in turn, would give astronomers a measure of the scale of the solar system. The Earth-Sun distance (or Astronomical Unit) is also key to the baseline measurement for the parsec, used to measure the distance to the nearest stars.
In a very real sense, Venus helped us unlock the first rung of the cosmic distance ladder.
Some of the greatest tales of astronomical adventure such as Captain Cook’s 1769 expedition to Tahiti came about in an effort to catch the transit of Venus. These were also the first expeditions undertaken in the name of science.
After this weekend, Venus transitions from an evening into a morning star, gradually reemerging low in the dawn sky. Venus reaches greatest elongation 46 degrees west of the Sun on June 1st, shining at magnitude -3.9.
May your very own neighborhood Venus expedition be as successful and adventurous. Be sure to track down Venus if you can this weekend, and follow its celestial exploits from one evening to the next. It’s a fun study in the very limits of what the human eye is capable of under the right conditions, and provides a great link to our astronomical past.