Planets and Perspective



Images Credit & Copyright: Daniel LaShomb (Me).  Feel free to use these images as long as you give proper credit and don’t sell or edit them.  Imaging techniques that I used are below.

On the night of June 30, 2015 the planets, Jupiter and Venus met up in the twilight of sunset in the western skies.  On the night sky they were separated by approximately one-third of a degree or just under a half-degree which presented a wonderfully awe inspiring sight.  In reality, Venus was 47 million miles from Earth while Jupiter was a distant 558 million miles from our eyes.  As I attempt to illustrate in these images (The Moon is added, it was not actually in the images), the two planets were close enough together to fit inside the apparent diameter of the one-half degree Moon.  If the Moon had been on course to occult the pair, it could have covered both of them at the same time.  That’s pretty awesome!

Leading up to that night they had been together on the sky for weeks.  The Moon even paid a visit on the nights of Friday, June 19th and Saturday the 20th creating a wonderful Isosceles Triangle; each night, the Moon allowing one of the planets to take a turn at the top.  Venus took watch on the 19th and Jupiter took the 20th.  A little further back, on Wednesday, June 12 Venus took up position next to the 577 light year distant Messier 44 (M44) the Beehive Cluster; an open star cluster in the constellation of Cancer the Crab.

You may have noticed as you were watching them that Venus is a lot brighter than the much more massive Jupiter and asked yourself why.  Or furthermore, how can an inferior (inner) planet be next to a superior (outer) planet on the night sky? And hey; why does Venus look like a crescent Moon?

Let’s start with the brightness.  Simply put, Venus is brighter than Jupiter because it’s so much closer.  On the night sky, Venus and Jupiter both had an angular diameter (How big it is on the sky) of about 32” arcseconds but Venus is

1: Extremely reflective.  Its albedo (reflectivity) is a whopping 70%.  That means it reflects 70% of the sunlight that hits it.  Compare that to the Moon’s 10%; can you imagine how bright the Moon would be if it were 70% reflective?  The only solar system body with a higher albedo is Enceladus with an albedo of around 90%!

2: Sunlight is about 50x more intense to nearby Venus than the distant Jupiter.

So even though Jupiter is 88,846 miles in diameter and can hold 1,000 Earth’s by volume; those two factors make the Earth sized Venus much brighter than Jupiter on the night sky.

So why is Venus a crescent like the Moon? Simple answer is; for the same reason.  Being an inferior (inner) planet, Venus never gets to the outside of Earth in our orbits to the only time there could even be a full Venus is when it’s behind the Sun and we can’t even see it then.  When it shows up on the western horizon as the evening star or “Hesperus,” it’s coming out from superior conjunction (behind the Sun) and it’s small and nearly full.  That’s because it’s far away (Further from us than the Sun) and sunlight is illuminating a nearly full disk as we see it from Earth.  As it catches up to Earth, its apparent size grows and it transitions from gibbous to crescent and that crescent gets thinner and thinner until it passes us on the inside and reappears in the eastern sky as the morning star or “Phosphorus.”  The process is then reversed as it moves away from Earth to once again go behind the sun, the crescent grows gibbous so we see more of the disk and it shrinks in overall size as it moves away.

How are they possibly together on the sky? This one’s not too hard to understand and just to help drive the visual home I added a solar system diagram from Popular Science below so you can see for yourself.  In short, Venus is closer to us and Jupiter is way off to one side of us which puts the two in the line of sight from our vantage point.



How’d I create these images?  Well first off, it was cloudy all night so it was patience that allowed me to catch the event.  I took the telescope (Celestron C8” on a CGEM) and my Nikon D750 out and got set up and waited for a window.  I just needed about a minute to jet a quick focus and test shots and another five for some imaging but what I got was a window about 20 minutes long; awesome!

The wide field shot is pretty straight forward as I positioned the camera over a lake and shot 4 seconds at 18mm at ISO 400 (it’s bright where I am and it was still twilight) and an aperture of f2.8.

For the tighter shots I’m not even going to list all my settings as it was all fly by the seat of my pants.  It took low ISO and a short exposure to find the crescent in Venus.  Otherwise as we detailed above, Venus is very bright and likes to be imaged as a ball of light.  For Jupiter I also had to shoot low ISO (But not as low as Venus) to keep the disk from being a bright white ball.  I’m disappointed that I didn’t really capture any bands though.  I’ll blame it on the atmosphere.

That low ISO to catch the disk of Jupiter is much too low to show the four Galilean moons so I had to bump the ISO back up and run a 10 second exposure to image them.  That of course overexposed the disk of Jupiter.

I then went to Photoshop and applied all three of the images.  I then kept the quality Venus, the quality Jupiter and the Galilean moons and erased everything else.  Just for fun, I added a couple renditions of the Moon to illustrate how close the two were on the sky.  Neither of these Moons that I compared them to were actually in the image nor was it in either of the two phases.  It’s just for illustration purposes.

Thanks All!




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