Conjunction Before Dawn
The two brightest planets had a close conjunction on August 12th. Three days before closest approach I took the following picture before dawn from my back yard patio at 5:44 am EDT on August 9th. Cloud free sky is rare here in southeastern Virginia, so I thought this might very well be the best conjunction image I would get.
Venus (magnitude -4) is the brighter dot above dimmer Jupiter (magnitude -1.9). The planets were separated by 2.9 degrees. At this point in their orbits Venus was 10.14 light minutes from Earth, and more distant Jupiter was 49.68 light minutes from Earth.
Much to my surprise, the sky was clear again at 5:44 am on the next morning, August 10th. In the next image you can clearly see planet spacing had diminished. Venus was still above Jupiter, but the separation had diminished to 2.03 degrees. During the previous 24 hours Venus had receded from Earth to a distance of 10.19 light minutes while Jupiter had come closer to a distance of 46.61 light minutes.
What is happening here? We are looking from Earth toward Venus and Jupiter across the plane of the solar system. Near dawn the Sun is below the horizon, and the planets are west of the Sun above the horizon.
Earth, Venus, and Jupiter all orbit the Sun and we are viewing the conjunction from a moving Earth. When viewed from above the solar system (looking down on the Sun's north pole) all the planets are orbiting counterclockwise. They're all in separate nearly concentric curved lanes. Rapidly moving Venus (on an inside lane) is moving away from our slower moving Earth (on an outside lane). Venus is beginning to approach the Sun from our viewpoint, and we'll eventually see it pass behind the Sun. So Venus is receding from Earth, getting smaller, and nearing the dawn horizon as it approaches the (below horizon) Sun.
On the other hand, Earth (on an inside lane) is overtaking slower moving Jupiter (on an outside lane). Jupiter is getting closer to Earth as Earth catches up to Jupiter. From our point of view Jupiter had recently passed behind the Sun and is now swinging around its orbit increasing its angular separation from the Sun. So Jupiter is getting higher above the dawn horizon as it separates from the Sun. Venus and Jupiter are heading in opposite directions relative to the horizon. Venus starts out above Jupiter, but, eventually, Venus sinks and Jupiter rises, so they switch places.
With naive optimism I hoped to show the daily decrease in separation over the next two days. Of course I should have expected disappointment. Annoying clouds ruined the view on August 11th! In the next image you can see a cloud gap. Unfortunately, Venus and Jupiter were located above the gap.
Clouds again interfered on August 12th, the day of closest approach. This time, fortunately, the cloud deck was thin enough to allow bright planets to shine through. In the next picture you can see brighter Venus beginning to drop below Jupiter. The angular separation had now decreased to 52 arc minutes or 0.87 degrees!
The plane of Earth's orbit projected against the sky is called the ecliptic. On August 12th Jupiter was nearly in Earth's orbital plane - only 3.83 arc minutes or 0.06 degrees below (south) of the ecliptic. Venus was 55 arc minutes or 0.92 degrees below the ecliptic. This difference in ecliptic latitude accounts for nearly all the 52 arc minute separation. What about ecliptic longitude? Jupiter's longitude was 104.1 degrees and Venus' nearly identical longitude was 104.28 degrees.
On August 12th Venus had receded further to 10.3 light minutes away and Jupiter had approached to within 49.46 light minutes of Earth.
August 13th viewing conditions were hopeless. A low impenetrable cloud deck blocked everything.
On August 14th conditions were similar to the previous picture, so it wasn't worth taking another picture of blank sky.
Before twilight on August 15th the dark sky was momentarily clear. I quickly took a photo and went inside to wait for twilight to grow. Only a few minutes afterward the sky became completely overcast! So the next image below was the best I could get on August 15th. You can see brighter Venus was now well below Jupiter. The angular separation had increased to 3.18 degrees. Venus had receded to 10.46 light minutes away and Jupiter's distance had decreased to 49.24 light minutes.
The image scale above is wider than all previous pictures. You can see how the entire Venus/Jupiter planet dance took place within the constellation Gemini. Gemini's twin stars, Castor and Pollux, are left of the bright planets. (Pollux is below Castor.) Orion was rising above trees on the right. If you don't get up before dawn, you miss beautiful scenes like this one. I get up so you don't have to!
Before dawn on August 18th the Moon joined the planet dance. I used night mode again to capture the next image with the same image scale as the previous picture. A neighbor's unfortunate spotlight interfered some, but not enough to ruin everything.
It would have been nice to follow the Moon's approach toward the planets over the next three days, but completely cloudy skies made this impossible.
To see how the planets moved without cloudy interruptions I used Sky Safari 6 Plus to create an animation running from August 9th to August 20th. The animation steps forward one day at a time to show the predawn scene at 4:44 am EDT each morning. (To see the video properly click the full screen icon in the lower right on the video.)
Notice how Jupiter lies almost exactly on the yellow line representing the ecliptic. The Moon makes an appearance in the last frame on August 20th making an attractive grouping with Venus and Jupiter. Also, notice how the stars get higher above the horizon each day as they rise approximately four minutes earlier each day due to Earth's orbital motion.
It's fun to watch the solar system in motion when clouds don't interfere.
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