Spotless

Saturn is high in the sky as the sun sets, reaching its highest elevation of about 60 degrees a bit after dark. The rings are wide open – well, for this year, anyway – at slightly over 4 degrees; as the month ends they’ll begin closing again, though we’ll miss them at their most edge on. They’ll start opening up again around November.

Pluto – still not overhead, but certainly visible this month – rises in early evening and transits a few hours after midnight. It’s not very high, though, only 35°.

All the other planets are in the morning sky. Venus, Mars, Uranus and Neptune all share the dawn, while Jupiter rises just after midnight. On the morning of the 17th (a new moon Saturday night) at a bit after 1pm, Jupiter will show a double shadow transit of Callisto and Io. We’ve had an amazing lack of those multiple Jovian moon transits, so it’s nice to see one that’s during halfway reasonable observing hours.

The RASC Handbook says there’s an occultation of Antares on the 10th visible in parts of North America – but nobody on the web lists it as being visible here, and XWphem shows the full moon skimming past Antares with a separation of a few arcseconds at about 2pm, when they’ll be well below the horizon. Not to knock the RASC – they do an amazing job of predicting interesting events more than a year ahead of time, so it’s fun to catch them in a rare error – but it looks like this one will be a non-event.

There’s one other shallow object to talk about this month – the really bright one that’s up during the daytime. Even veteran solar observers haven’t been observing the sun much lately ... because there’s nothing to see. It’s not too surprising, since we’re just a little past the “solar minimum”. That’s the time when the sun, a variable star, typically shows the least activity: the fewest sunspots to observers with white-light filters, and the fewest flares and prominences to people with H-alpha filters. The typical solar cycle lasts 11 years.

But the end of that 11-year cycle was last year, and still solar observers are seeing no activity. There’s so little that it made the news last month, when there were 15 days without a single observable sunspot. Last year had 266 days without a sunspot, as well as over 85% of days without a smudge so far in 2009 – quieter than it’s been for a hundred years.

What are sunspots, anyway? Basically, they’re slightly cooler areas on the sun’s surface, caused by regions of high magnetic activity. They’re tied to other solar activity like prominences (those lovely loops you can see with an H-alpha filter), and they also cause interference with radios and other devices like GPS receivers here on earth.

Sunspots may even affect the earth’s weather. Paradoxically, when a lot of sunspots are present, even though the spots themselves are cooler, total solar radiation increases. Some sources say that’s because the halo around a sunspot is hotter than the average solar surface, while others blame it on particles ejected because of the intense magnetic activity (or perhaps these two reasons are related). Whatever the reason, when there are no sunspots, our weather often gets a little cooler. The “little ice age” of the 17th century corresponded with a long period of no observed sunspots, called the “Maunder Minimum”. But not everyone is convinced of the correlation, and there have been other periods of extremely low activity (in the early 1900s) without any obvious weather correlation.

The sun is definitely cooler now than at the last solar minimum in 1996. It’s only .02% dimmer in visible light, but it’s lost 6% of its UV radiation since 1996. Don’t put away that sunscreen just yet, though – 6% isn’t that big a difference.

Of course, if you have an axe to grind you can draw all kinds of conclusions about global warming from the dearth of sunspots. Global warming is worse than we think, because the low solar activity is masking the warming effects! No, it’s not as bad as they say, because the sun is now entering a cooling phase and we’ll see more cooling over the next few years! Truth is, nobody knows for sure what the sun’s going to do, and global warming probably doesn’t have anything to do with the sun.

But however you look at it, solar observing is going to be boring for a little while. So put your solar filters in storage for a little while and find something else to observe during the day. Maybe even something that won’t affect the climate. Me, I think I’m going to leave the car in the garage for a day, dust off that bicycle and go for a ride. But wait – first let me put on some sunscreen.