Early observers of the night sky thought that Earth was at the center of the universe, but now we know better. There is no center of the universe. No matter, in many ways we continue to act as if we’re at the center of the universe. Many of the measures that we talk about as amateur (and professional) astronomers are based on circumstances that are unique to us, Earth, and our solar system.
By Bill Pellerin
Houston Astronomical Society
GuideStar Editor
Did you see it? Did you see it?
After reading a couple of books about the history of the Venus transit I was very eager to see the one on June 5, 2012, and I did. The most recent book I read was The Transits of Venus by William Sheehan and John Westfall. This book tells the history of Venus transits since the first one known to have been observed by Jeremiah Horrocks and William Crabtree in December of 1639. It’s a remarkable story of determination by those observers who timed the next pair of transits in 1761 and 1769 on the recommendation of Edmund Halley. Halley died before either of these transits. The goal was to determine the distance between the Earth and the Sun, which they did but without the hoped-for accuracy.
I began to wonder about the mechanics of the event. How were Venus and the Sun arranged in the sky, and how were they moving during the transit? Did the movement of the Sun or of Venus contribute more to the relative movement that I saw?
We’ve learned that these events are rare because the orbital planes of the Earth and of Venus are not the same. Only when the planes cross each other while Venus is at inferior conjunction does a transit occur. An inferior conjunction is when the orbit of an ‘inferior’ (closer to the Sun) planet is between the superior planet and the Sun. The next inferior conjunction will be on January 10, 2014, but Venus will miss the Sun by about 5 degrees, and there will be no transit. The next transit is in 2117.
Let’s do a replay of the June 5, 2012 event to understand what was going on. Observationally, we saw Venus attack the north-eastern edge of the Sun and then move slowly westward across the face of the Sun. For the purposes of this article, the movements are considered from the point of view of an observer on the Earth.
When Venus is at inferior conjunction the apparent motion of the planet is to the west with some motion to the north or south. I simulated all eight transits back to May 23, 1526. For every May or June transit, Venus’ apparent motion was to the southwest, and for every December transit Venus’ apparent motion was to the northwest.
What was the Sun doing? The Sun is reliably moving east in the sky (compared to the background stars) year-around with some additional movement to the north or south because of the tilt of the Earth’s axis. You only have to know about the motion of the Sun during the four seasons to understand the motion of the Sun during a transit. In spring, the Sun is moving to the north and it continues to do so until the summer (northern hemisphere) solstice (the first day of summer – June 20, this year).
So, while the Sun was moving mostly east, it was also moving a bit north during the most recent transit and the position angle of the Sun’s motion was 82 degrees, just north of due east. Remember that north is 0 degrees, east is 90 degrees, south is 180 degrees and west is 270 degrees. The Sun moved 15’ 50.86” east and 2’ 13.6” north during the time of the transit the total motion of the Sun was 16’ 2”.
Until about May 13, 2012, Venus was moving mostly to the east in the sky. Between that date and about June 26, 2012 Venus was moving to the southwest at a position angle of about 244 degrees. After June 26, Venus continued its eastward trek in the sky. During the transit Venus moved 10’ 06” west and 4’ 56” south, the total Venus movement was 11’ 14.3”.
The movement of the Sun and Venus were in more-or-less opposite directions, but not exactly. If you do the vector math the combined movement is 26’ 55”.
So, if you want to see the replay, and you have software on your computer that simulates the positions of Venus and the Sun accurately, you can set this scenario up and watch it play out. We’ve been told that we won’t see another Venus transit in our lifetime. That’s true, but we can watch the replay of the event on our computers as many times as we want. Johannes Kepler, Edmond Halley, and their contemporaries would have been astonished by this capability.
After the 2012 transit ended, I thought about the observers who will see the next transit in 2117 and I wondered what tools they will have at that time. Surely it will be an amazing time, and some of those observers will be thinking about the observers of the previous transit in June of 2012 and wondering what the event was like for them.
Thanks to fellow Houston Astronomical Society member Bill Flanagan who contributed calculations to this article. (Any mistakes are mine.)
(Kansas City, MO)–The Astronomical League is pleased to announce the top finishers in the competition for its Jack Horkheimer Service and Journalism Awards. The Horkheimer Service Award competition is open to students under 19 and students 8-14 are eligible to apply for the Horkheimer Journalism Award. All top finishers in the Jack Horkheimer Award program receive $1,000 in addition to a beautiful award plaque recognizing their special accomplishments. More information may be found on the Astronomical League website test.astroleague.org.
The first-place winner of the 2012 Jack Horkheimer/Smith Award is Benjamin Palmer from Queensbury, New York. Since the age of nine, astronomy has been his passion. He applied for and was accepted as an intern at Dudley Observatory. While at Dudley he participated in many large and small outreach events, with his enthusiasm for astronomy carrying over into organizing star parties for local high schools and the 4-H club during this internship. Among his many activities, he is currently developing astronomy education software for classroom use and investigating the possibility of conducting virtual online star parties. He is a member of the Albany Area Amateur Astronomers, Inc.
Benjamin has won an expenses paid trip to the national convention of the Astronomical League, ALCon 2012, being held in Chicago, IL July 4-7, where he will be speaking about his astronomical experiences and receiving the award.
The first-place winner of the Jack Horkheimer/Parker Award is Samantha Carter who lives in Fairview, Texas. Samantha has been active in giving service to her astronomy club, the Texas Astronomical Society, and to the Texas Star Party, as well as coordinating astronomical activities for her own girls scout troop. Currently she is working on the Astronomical League Messier and Urban Observing Awards.
The first place winner of the Jack Horkheimer/O’Meara Journalism Award is Katelyn Skaer from Roswell, Georgia. Her award-winning essay is entitled “A Star is Born.” Katelyn is a member of the Atlanta Astronomy Club.
About the Astronomical League
The Astronomical League is the largest group of amateur astronomers in the world. Its membership numbers 15,000 from over 250 clubs and individuals. The mission for over 60 years has been to promote the science of astronomy by fostering astronomical education, by providing incentives for astronomical observation and research, and by assisting communication among amateur astronomical societies. The organization is a 501(c)(3) non-profit entity.
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.
On Thursday, Feb. 9, beginning at 9 pm PST, 8 pm MST, 7 pm CST, 6 pm EST Dr. Soifer will present some of the most exciting results from NASA’s Spitzer Space Telescope. Among its many discoveries include detecting emissions from hot Jupiters orbiting other stars, finding Fullerene (carbon buckyball) molecules in space, observing prebiotic molecules in galaxies seen less than 3 billion years after the Big Bang, and measuring the masses of the most distant and youngest galaxies known. Dr. Tom Soifer is a Keck Observatory board member, Director of the Spitzer Science Center and Chair of the Division of Physics, Mathematics and Astronomy at Caltech.