Waiting for the Great Red Spot

It takes a confluence of events to make a good evening for the telescope and last night delivered just that. By mid-afternoon, I had noticed the clear blue sky outside my window as I worked, and, just to be sure, I consulted the Clear Sky Clock for my area.

The Clear Sky Clock predicts viewing quality using four separate forecasts. Of great importance of course is the lack of Cloud Cover. You want to be out on a night where the moon isn’t visible for optimal Darkness. Those two criteria are fairly easy to predict, with the latter being merely mathematical and the other covered on your local weather report. The last two parts of the Clear Sky forecast are more unusual. Transparency in the atmosphere depends on the amount of water vapor in the air. Even clear skies can contain a high amount of water vapor and lead to a less than satisfactory viewing night. Rounding out the forecast is Seeing, a factor affected by turbulence and temperature differences in the atmosphere. You’re seen how things viewed through the turbulent air above hot pavement seem to shimmer? The same effect causes objects viewed through a telescope to be unclear.

Last night, all four factors were favorable in my area, which created a good night for astronomical viewing. In my yard, I have another issue to contend with. I only have a clear view of about a third of the sky, with the rest being obscured by trees. Fortunately, one of the premium astronomical targets currently spends much of the evening in a prime viewing location from my yard. That target is the planet Jupiter.

With good viewing conditions forecast, I was hoping for a chance to observe Jupiter’s Great Red Spot. The GRS is a huge storm visible in the cloud patterns on Jupiter. Since Jupiter rotates around its axis in just under 10 hours, it’s important to know the times when the spot will be in view. In particular, the best times to view the spot are around the times when it transits across the ‘central meridian’, which is the midpoint of the face of Jupiter that we see.

I used an online tool to check the time for the next transit of the Great Red Spot, and I was pleased to find that a transit would occur around 12:40 AM. I set the scope up in early evening and enjoyed viewing a variety of objects in the sky while waiting for a chance to get my first glimpse of the GRS. My favorites for the early evening were the globular star clusters Messier 3 and Messier 5. At low power, these look like hazy round areas of light, but with a higher power eyepiece, you can distinguish some of the individual stars around the edges of these clusters which are thought to contain half a million stars.

As midnight approached, I started trying out various combinations of eyepieces and filters to see which gave the best view of Jupiter’s cloud bands. Since Jupiter is a very bright object in the sky, I found it easier to view through a filter that blocks much of the light. Once I got accustomed to the view and started looking, I was able to see the Great Red Spot and watch it move across the face of Jupiter for about an hour. Very cool!

All in all, it was a great night. My only complaint was the attack of the mosquitoes, who decided they wanted to participate as well. Jupiter and my first view of its Great Red Spot are well worth a few mosquito bites though.

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