All image models of the solar system in this post were created using the software on www.solarsystemscope.com
An Introduction to Venus
When visitors at Sidewalk Science Center ask "What's that bright star that's always by the Moon?" I tell them it's the planet Venus, then I often joke, "I like to say that people only notice it when the Moon is by it."
In a way, it's true. After the Sun, the Moon is the brightest object in the sky and easy to spot when it's on the evening side of the Earth. So of course people will suddenly take notice when another bright dot is next to it, thinking "Huh, that wasn't there yesterday!" Except if you've glanced up at the western sky after sunset at any point since December 2022, Venus has been there...every single night!
Venus circles the Sun every 225 Earth days, or about once every 9 Earth months. Interestingly enough, this nine-month period is just longer than the same amount of time that Venus is visible to the naked eye on Earth during its evening or morning cycles, hence the nicknames: the "Evenstar" (western sky post sunset) and the "Morning Star" (eastern sky pre sunrise). This is all to say, from its appearance out from behind the Sun in mid December 2022, to its disappearance in front of the Sun in early August 2023, Venus will have completed almost one full orbit, or one Venusian Year.
So if you watch Venus during its entire visible Evenstar period, then during its entire visible Morning Star period, you will have watched for just shy of 18 months, or two Venusian years.
NB: when referring to "visible to the naked eye" or "visible appearance," I mean since Venus exited the glare of the Sun. There's a period of time where it's not behind the Sun, but not visible due to the bright light being scattered within the atmosphere.
The Phases of Venus
At Sidewalk Science Center, we've been showing Venus through our telescopes since its visible appearance in December. At that point, Venus was almost directly across the solar system from Earth, roughly 160 million miles away. As Earth and Venus both travel in elliptical (near-circular) orbits, Venus has been moving off to the side of the Sun relative to our geometrical view from Earth. Note that Venus is not actually increasing its distance from the Sun by any noticeable amount to the human eye, just the angle between Earth, the Sun, and Venus is changing, causing Venus to appear to move away from the Sun.
When Venus is still mostly behind the Sun, let's say between the angles of 0 and 45 degrees with Venus as the angular vertex, it appears in a phase we call a "Full Venus." Viewing Venus through a telescope, you will see a mostly-circular white dot with a measurable angular diameter and surface area: the shape of the planet!
Venus continues moving off to the side of the Sun, let's say between the angles of 45 and 80 degrees, it appears in a phase we call a "Gibbous Venus." Viewing Venus through a telescope, you will see a dot with a slightly larger angular diameter and surface area. However, it will appear squished, more like an oval than a circle.
As Venus nears a 90 degree angle with the Earth and Sun, it will take on the phase of appearing as a "Half Venus," or a little more appropriately, as a "Quarter Venus." Looking through a telescope, Venus will appear as a tiny D shape, and one we're all familiar with: it looks like a tiny Half (Quarter) Moon!
The Elongation of Venus
When Venus reaches a 90 degree angle with Earth and the Sun, it is now at the farthest point away from the Sun as viewed from Earth. In astronomy, this is called the Point of Elongation; that is to say, Venus will stop moving away from the Sun, and start tracking in the opposite direction, back toward the Sun.
In 2023, the date this elongation occurred was June 4th. Viewed on this date, Venus looked like a perfect half circle, with an angular diameter more than 2x compared to that measured in December, and about 4x the measured surface area.
The Crescent Venus
Now that Venus has passed its point of elongation, the shape of the light reflecting toward the Earth will begin to shrink, and form a crescent in exactly the same way we view a crescent Moon.
As the angle between Earth, Venus, and the Sun (with Venus as the vertex) grows from 90 to 180 degrees, the light reflected off the planet will point more and more away from Earth, providing us a thinning sliver of light that becomes sharper and easier to see when viewed through a telescope.
Although the light itself is thinning, you will also see that the angular diameter, and by extension, the surface area, of Venus grows wide and larger. This is because Venus is now starting to move between Earth and the Sun. It's closest approach to Earth takes it within about 25 million miles of our planet. Compared with the angular diameter of its farthest visible point 160 million miles away, the diameter of Venus by the end of July 2023 will be about 6x wider than the measured diameter in December 2022, equating to a roughly 36x larger surface area.
However, due to the thinning crescent of light, the surface area of light does not match to the surface area given by the angular diameter. At this point of proximity, we will mostly be seeing Venus' nighttime side, so the calculated surface area has to be inferred from the fact that a solid object is still there.
In Conclusion...
From our relative view on Earth, Venus spends about nine months in the post-sunset sky, and about nine months in the pre-sunrise sky. During each of those nine, it spends about seven months as a Gibbous Venus (squished-to-mostly circular) and only two as a Crescent Venus.
Remember, while it orbits the Sun faster than Earth does, Earth itself is also moving and so travels some distance before Venus becomes visible from/vanishes into the glare of the Sun. Those four total months it spends as a crescent, it is speeding past the Earth, so quickly drops from its elongation point out of the western sky followed by quickly rising up to its elongation point in the eastern sky.
Currently, in early June 2023, we are at the point where Venus is now speeding back "toward" the Sun. So visit Sidewalk Science Center from now until the end of July to witness the spectacle of a crescent-shaped Venus in a telescope, and realize for yourself the awe-inspiring geometry of our solar system.
To all our endeavors,
Alex Martin
Executive Director, Sidewalk Science Center