By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

As I write this I’m preparing for the June 5, 2012 (at my location) transit of Venus. My first goal is to be able to see it, my second goal is to be able to photograph it, and my third goal is to make the observations necessary to get the Astronomical League award for the transit. For details on the event and the award see the ‘Transit of Venus’ page in the ‘Observing Programs’ on this web site.

http://astroleague.org/PlanetaryTransit_Venus2012

What do I need to have in place to reach these goals? I need the necessary telescopes with the right filters, eyepieces, and camera fittings among other things. I need to have tested the various configurations I’ll use for the equipment, and I need a plan for the day. In short, I need to be prepared.

Strategically, I plan to be in a location in which the chances for clear skies are as good as I can reasonably make them. By ‘reasonable’ I mean that it must be a location that I can conveniently get to by car (so I can haul all my stuff along) and the chance of clear skies must be high enough to justify the effort. So, in the end, I’m trying to balance the improved opportunity to see the transit against the cost of that opportunity.

The problem is, though, that there are no guarantees about the weather; I’m at its mercy. An 80% chance of clear skies means a 20% chance of cloudy skies. That said, a site that only has a 20% chance of clear skies may, in fact, have clear skies on the day of interest and a site that has the 80% chance of clear skies may be cloudy on the day of interest. By studying the weather maps carefully on the morning of the event it may be possible to change my observing destination to improve the odds of making the observation. I’ll be looking at the current cloud cover and the direction of movement of the clouds. I want to be behind the path of the clouds, in the location that looks like it is tending toward clear as the day goes on. There are plenty of weather reporting sites on the Internet, and there’s every astronomer’s favorite site, the Clear Sky Chart (cleardarksky.com). Even with this effort, it’s no sure thing. It’s possible to have cloud cover for hundreds of miles in any direction.

This is the problem with ‘event’ astronomy. By that, I mean an event where you must be under clear skies at the right location and at the right time in order to make the observation. The most common example of event astronomy is a solar eclipse. For one of these, you have to be along the line of the eclipse, at the right time, and under clear skies.

How many hard-luck stories have you heard (or told) about being in position, and on time to see a solar eclipse only to miss the observation because of clouds? The July 22, 2009 solar eclipse was clouded-out from many locations in China and many astronomers who had gone there to see the eclipse were disappointed. There were many tales of woe on the Internet following this event.

I went to Mazatlan, Mexico for the July 11, 1991 solar eclipse, and I saw totality, but not well. The skies were hazy so the view was less than perfect. A few observers jumped into cars seeking clear holes in the sky for totality, but in the end they didn’t do much better. You never know.

Occultations are like eclipses, in that one object blocks another, but the term is more often used for other events – the moon blocks a bright star or another planet, an asteroid moves in front of a star and causes it to wink out, a moon moves behind a planet (often Jupiter or Saturn), or (rarely) a planet moves in front of another planet. These all require clear skies at the time of the event for you to observe them.

Professionals have a similar problem. For the large, ground-based professional telescopes, there is a committee that schedules time on the instrument. If you schedule your observation and the skies cooperate, you get your data. If the skies fail to cooperate, you don’t, and you have to reschedule. If you get time on the Hubble Space Telescope, clouds will not be a problem.

If you want to read about an epic series of disappointments, read about a Frenchman commonly known as Le Gentil. He and his team were tasked with timing the 1761 Venus transit, but that observation failed. Not giving in, Le Gentil knew that there would be another transit in 1769, so he tried again, and failed again. All in all, he spent about 10 years on the task and had no results to report. When he got home to France after 11 years he found that he had been declared dead and his possessions had been sold. Now, that’s a bad observing trip!

I wish you the best of luck with your observations – especially those associated with an astronomical event.

Clear skies for all of us!

(Kansas City, MO)–The Astronomical League is pleased to announce the top finishers in the competition for its National Young Astronomers Award Program (NYAA).

The first-place winner in the NYAA program is Justin Tieman from Lee’s Summit, Missouri. His project was entitled “Alien Worlds.” The hypothesis of this experiment is that an amateur astronomer may be able to detect an exoplanet with only a backyard telescope and using differential photometry software. His conclusions verified this hypothesis. Also a second project “Space Rocks” involved precise light measurements in asteroids.

The second-place winner is Travis Le who lives in Aica, Hawaii. His work “Determining ‘Hot Spots’ Through Correlations of CMES and Solar Flares” was able to determine five "hot spots" on the Sun where possible CMES(Coronal Mass Ejections) could occur.

Brian Graham from Beaverton, Oregon is the third-place finisher. His project attempted to answer the question regarding whether telescope tracking errors had any effect on the accuracy of exoplanet light curves. These results support the hypothesis and imply the effectiveness of the Exoplanet Transit Tool.
The top two finishers have won an expenses-paid trip to receive their awards at ALCon, the national convention of the Astronomical League, being held in Chicago, IL7-4 thru 7-7-2012.
 

By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

I was checking the weather forecast earlier this week. The weather web-site that I consulted contained additional information, including sunrise and sunset, and moonrise and moonset. It caught my attention that in a couple of days there would be day without a moonset. Interesting. Probably most of us think of the moon rising and setting on a daily basis, and on most days it does just that. After consulting the US Naval Observatory web site showing moonrise and moonset for my location I realized that there is one day a month without a moonrise and one day a month without a moonset. Actually, there’s one day per lunar cycle without a moonrise and one day per lunar cycle without a moonset.

Let’s start at the beginning and see if we can figure out why this happens. The objects that are important in this analysis are the moon (obviously), the sun, and the earth. When the moon is at the same RA (right ascension) as the sun we have a new moon and from our point of view on earth we don’t see any illuminated part of the moon. RA is the coordinate system in the sky that corresponds to east/west. Declination is the measure of the distance of an object from the equator, so it measures distance north/south. If the new moon happens to be at the same RA and the same declination as the sun, we have a solar eclipse. When the moon’s RA is 12 hours different from the sun’s RA, we have a full moon. The earth time from new moon to new moon is 29.53 days (29 days, 12 hours, 43 minutes). This time takes into account the apparent movement of the sun over that time.

These numbers get modified because the moon’s orbit is not circular, it’s elliptical, and keen-eyed observers of the moon will be able to see the difference between the moon at apogee (farthest from earth) and the moon at perigee (closest to earth). This is in part why some solar eclipses are annular and some are total.

We know enough to figure out why there’s no moonrise on one day per lunar cycle and why there’s no moonset on one day per lunar cycle. Because the moon makes one trip around the earth (relative to the sun) in 29.53 days, we can say that the average angular distance traveled per cycle is 360 (re the sun)/29.53 which equals 12.2 degrees. (Strictly speaking there’s another one-degree due to the movement of the earth on its orbit, but we’re close enough.) Since the earth rotates 15 degrees per hour (360/24) we can easily ask how much later, per day, are lunar events (moonrise and moonsets). The answer is (12.2/15) * 60 = 48.8 clock minutes. This is certainly in the ‘ballpark’; I’ve found some references that say the difference is about 49 minutes per day and other say it’s about 50 minutes per day. To keep it easy, let’s go with a 49 minute per day difference.

If the moonset on any particular day is more than 49 minutes before midnight, the next moonset will occur on the next day. However, if the moonset on our first day of interest is less than 49 minutes before midnight, there will be no moonset on the next day; the next moonset will be just after midnight on the third day.

This is what a professional astronomer would call a back-of-the-envelope calculation. It doesn’t take into account a lot of details that would be important if an exact determination of the timing of this phenomenon is needed. I can think of several considerations that were left out – angle of the moon to the horizon, apogee and perigee of the moon, latitude of the observer, and probably a few other things that don’t occur to me.

How does this fit with real data? To see, I analyzed data from the Naval Observatory moonrise / moonset chart for all of 2012. Based on this calculation I’d expect that the time between successive moonsets is 24 hours 48 minutes and 46 seconds. I looked at the data for 2012 for my hometown (Houston, TX) and found that the average time between moonsets for over 300 moonsets was 24 hours + 50.4 minutes. I’m in the right ballpark. The actual difference in moonset time on any given day can vary quite a bit. At my location the minimum difference in 2012 is 35 minutes and the maximum is 68 minutes.

If you look at a moonrise / moonset calendar you’ll see that the day with no moonset is near the first quarter and the day of no moonrise is near the last quarter. If you think about the geometry of this, it makes sense.

It’s fun to say to your astronomy friends, “Guess what? There’s no moonset next Thursday.” You’ll probably get a blank stare back.

You want to see when you’ll have a day without a moonset (or a moonrise)? Go here:

http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-one-year-us

…enter the year, specify that the ‘Type of Table’ is moonrise/moonset, enter your location, and click [compute table] and you’ll get a table of all moonrise and moonset events for your location. For locations outside the US, you can enter the latitude and longitude of the location. Note that the table does not take into account daylight saving time. So, if you observe daylight saving time, or the equivalent, be sure to compensate for that.