Spots in the Eyepiece

Tempestuous. That’s an apt description of gas giants Jupiter and Saturn. They are both the right mass, size, and have a predominantly gas structure. Balls of hot air, cold air, and blustery winds. On Earth that describes politicians, eh?

We have known about the Great Red Spot (GRS) on Jupiter for a long time. While the first observation of the GRS is not known for sure, astronomer Giovanni Cassini’s report of a “permanent spot” on Jupiter in 1665 is considered the most likely candidate. Bear in mind that the telescope was invented a mere 57 years prior to Cassini’s observations. Telescope making technology had not progressed significantly. The design was plagued by what is called spherical and chromatic aberrations, making the viewed image hazy and difficult to focus. It wasn’t until 1688 that a design with fewer aberrations was developed, by Issac Newton. He used a mirror instead of glass lenses. The mirror produced much less spherical error and no chromatic error, so its image was superior to the refractive telescopes at the time. Refracting telescope design improved dramatically in 1733 with the invention of the achromat lens design.

Observing the presence of a GRS on Jupiter became hit-and-miss over decades, so we aren’t even sure if today’s GRS is the same one as from the 17th and 18th centuries. Nevertheless, the GRS, we know, is a monster cyclone – our Earth could fit inside.

If you examine the outer planets, starting with Jupiter, their atmospheres appear to get progressively calmer. Saturn has visible zones and belts, albeit way more subtle than Jupiter’s. Uranus’s and Neptune’s atmospheres  are flatter still, with little definition visible. A big part of the reason is that each planet beyond Jupiter is progressively smaller, with decreasing mass and greater distance from the Sun.

When observed with a telescope today, Saturn at first appears bright, pale yellow,  with no bands, but with magnificent rings. At higher power and after some concentration Saturn’s belts and zones come into focus. While Saturn does not have a Great Red Spot, it has white spots, some quite large. And the white spots are different in composition compared to the GRS.

Then there’s the Great White Spot (GWS). The first recorded observation of a GWS was in 1876, over a hundred years after Jupiter’s GRS. That’s understandable, as telescopes needed to get bigger and better – Saturn is nearly twice the distance from the Sun as Jupiter.

The GWS differs from Jupiter’s GRS in that the GRS is calm in its center, the GWS is still very turbulent. The GRS’s reddish color is thought to come from ammonium hydrosulfide and acetylene’s interaction with solar ultraviolet radiation. A GWS is essentially due to water going through solid, liquid, and gaseous phases during seasonal changes, and is not permanent, but it’s visible in backyard telescopes.

What’s in the Sky?

August 30; 1 hour after sunset; east-southeast: A BLUE + Supermoon is on the rise, just southeast of Saturn.

Blue Moon: 2nd full Moon in the same month. Supermoon: Full Moon during perigee (the Moon’s closest approach.)