A conjunction occurs when two celestial objects are in line as seen from the Earth. The inner planets, Mercury and Venus, can have two different conjunctions with the Sun. These are called superior conjunction, when the planet is on the far side of the Sun, and inferior conjunction, when the planet is on the near side of the Sun. This month we will witness a nice inferior conjunction of Venus as it passes from the evening sky to the morning sky.
One of the most useful features of Starry Night® is the ability to view the solar system from different perspectives. This can help us understand what goes on at an inferior conjunction of Venus. First let’s get an overview of the process. Under the Favorites menu choose Solar System:Inner Planets:Inner SolarSystem. This gives you a bird’s eye view of the four inner planets, seen obliquely. Click the Time Stop button and then click on the menu at the right end of the Time and Date box and choose “Now.” Zoom in until the Earth’s orbit fills the window. Choose the “Location Scroller” cursor from the Cursor box, just to the left of the Time and Date box. This lets you move your viewing location by dragging on the screen. Drag downwards so that you get a view looking straight down on the solar systems, so that the planetary orbits appear circular. Note the position of Venus somewhat above the imaginary line joining the Earth and Sun.
If you now advance the date one day at a time by clicking on the date and then pressing the up cursor key on your keyboard, you’ll see Venus gradually move into line with the Earth and Sun. The three will line up perfectly on March 27, which is the date of inferior conjunction.
You may wonder why Venus doesn’t actually pass in front of the Sun. Use the location scroller to drag upwards on the screen until the Earth’s orbit (green line) becomes a straight line. You’ll notice that Venus is now well above the plane of Earth’s orbit because of the different tilts of the two orbits. This means that for observers in the northern hemisphere, Venus will pass above (north of) the Sun. This gives a special advantage in observing Venus which we’ll get to shortly.
Now lets look at how the conjunction appears to an observer on Earth. Use the menu button at the far right end of the Location box to choose your regular viewing location “with Reset.” Stop the time advance and set the time to 12:00 noon. We want to see clearly what’s happening around the Sun, so turn off the halo effects. You do this by right-clicking (Windows) or control-clicking (Mac) on the Sun to open the contextual menu, and choosing Halo Effects:Never. Under the Labels menu, choose “Planets-Moons”.
Again, advance the date one day at a time by clicking on the date and using the up-cursor key. Notice how the Sun moves northward in the sky (spring is coming!) and how Venus moves to the right (northern hemisphere) and passes well above the Sun. On March 27 Venus is at its closest to the Sun, just above our line of sight. In the sky, this takes place when Venus is somewhat to the right of the Sun, because of the tilt of the ecliptic in the sky.
Wouldn’t it be interesting to see how Venus is illuminated as it passes above the Sun? It’s easy with Starry Night®! Go back to the current date by choosing “Now” from the Time and Date box menu. Stop time as before and set the time to noon. Starry Night® lets us enlarge Venus (or any other planet) in the sky, while keeping the sky itself at normal scale. First, I prefer to see Venus with a cloudy surface, rather than the default, so right-click (Windows) or control-click (Mac) on Venus, and choose “Surface Image/Model:Clouds”. Now let’s magnify Venus. Click on the Find tab on the left side of the screen to open the Find pane. Venus should be the third item down. Use the scroll bar at the bottom of the Find pane to scroll to the right until you see a column headed “Magnification,” which may be abbreviated to “Mag…” This is a column of “sliders” which let you enlarge that particular object. Slide Venus’ slider slowly to the right, and you will see Venus gradually enlarge! Enlarge it so you can clearly see its phase and the “ashen light”: the part of Venus which is unilluminated by the Sun, but which is actually slightly lighter than the sky background.
Now advance the date one day at a time and watch what happens to Venus’ phase. It shrinks to the thinnest crescent, and moves around to the south side of the disk as Venus nears conjunction on March 27. Continue to advance the date past conjunction and watch the phase grow and shift around to the eastern side of the disk.
It is actually quite easy to observe Venus’ phase near conjunction with a small telescope or even binoculars. As always when observing close to the Sun, you have to take precautions. Because Venus is passing above the Sun, it is very easy to locate yourself so that the Sun is safely hidden behind a roof peak or chimney. Starry Night® will show you the direction and distance from the Sun for any particular day. I have observed Venus myself within about a day of inferior conjunction with an 8-inch reflector telescope. The cusps of the crescent were greatly extended beyond the 180° you would expect, and the ashen light was clearly visible. This month is an especially good opportunity to observe this because Venus will be relatively far from the Sun, over 8 degrees. Clear skies!
Geoff has been a life-long telescope addict, and is active in many areas of visual observation; he is a moderator of the Yahoo "Talking Telescopes" group.