The moon, bright , entrancing, intensely irritating for astronomers. Scattered moonlight makes observing more inefficient meaning astronomers have to stick on a target for longer. Observatories are expensive, high-tech facilities so the question is, how much does the Moon cost astronomy?
Astronomers are in essence counters. Telescopes collect photons from a source and astronomers count the number in their detectors. The telescopes also collect photons from the night sky. This is never 100% dark so there are also some additional background photons which must be subtracted off. As with all counting there are uncertainties. The fainter the object the higher the uncertainty, the brighter the sky the higher the uncertainty. The Moon is basically a big mirror reflecting sunlight into telescopes. As the amount of sunlight the Moon reflects towards the Earth varies over a lunar cycle, the sky background is much higher at Full Moon than at the New Moon. This means there is a much higher uncertainty in astronomical measurements at the Full Moon so astronomers have to observe for longer and get more signal to have a measurement as certain as one from a shorter observation taken at New Moon.
So how much observing time does this cost and how much is that in terms of money? I’m going to use some very rough estimates based on publicly available numbers. If these are off by a bit, feel free to correct them in the comments.
So firstly how much time does this cost? Astronomers typically describe nights as dark, grey or bright depending on the phase of the Moon. Let’s assume that for 50% of the time when the telescope is open it is integrating in the optical*. This takes into account overheads such as slewing and that some of the time the telescope will be observing in the infrared. The wavelength is important as the amount of reflected sunlight varies with the colour of the filter you are observing through. Bluer, shorter wavelengths are typically more seriously affected by the Moon. Hence let’s ignore the effect on infrared observing. Picking a typical optical observation band (the R band) which is not really badly affected by scattered moonlight I had a look at some Integration Time Calculators. These are tools which allow astronomers to work out how long they have to observe a source for. To reach a particular uncertainty of observation you need to integrate for 60% longer in bright time than dark time and 5% longer in grey time than dark time. So assuming 1 week of bright time per lunar cycle plus one week of dark time and two of grey time. That means that astronomers lose about 6% of telescope time due to having to observe for longer in bright and grey time taking into account our 50% overheads/IR observing factor.
So how much money is this? Well the Keck Observatories (two 10m telescopes) have annual budget of $11m. But that is only the running cost, what about the construction cost? The VLT in Chile (four 8m telecopes) cost €330m in 1999 to construct. Converting to 2011 dollars that’s $650m. Assuming a 40 year lifetime for the telescopes about $16m per year is spent on construction. So $4m per annum for one 8m telescope and $5.5m running cost for one 10m telescope. Let’s assume these numbers are typical for one 8m class telescope. There are 16 telescopes of 6.5m diameter of larger, assuming all these have a $9.5m annual cost and ignoring other telescopes, that comes to $150m spent annually on large telescopes. Six percent of that time is taken away by the Moon at a total annual cost of $9m.
So that’s a very rough number for the annual cost to astronomy of the Moon. This was just for fun so I don’t expect it to be correct to the last cent but hopefully to an order of magnitude. Any better estimates are welcome in the comments section.
* Yes I know infrared observations are more likely to be scheduled during bright time.
If you live in the continental US and you want to see a solar eclipse then Monday 21st August 2017 may be your lucky day. The path of totality will stretch narrowly across around 11 states from Oregon to South Carolina, and the rest of North America will see a partial eclipse instead.
The combination of location and probable good weather means this eclipse is likely to be seen by many thousands of people. People with digital cameras. Pretty good digital cameras with reasonable optics and CCDs. What if all those pictures could be collected and combined together? Well, that’d give some lucky solar physicists a massive, long, high-resolution, continuous movie of the solar corona, showing in detail how it evolves over the 90 minutes totality lasts over the US. It’d also be a pretty good outreach project too, with many opportunities for getting the public involved.
This is the thinking behind the Eclipse Megamovie, first put forward at the American Astronomical Society meeting in June and followed up last week in a paper published on the astronomy preprint server. The project is still very much in the planning stage, with many issues still to be sorted out (including the technical difficulties associated with handling, processing and combining the flood of data involved), but it’s an interesting idea, and one I hope to hear a lot more about over the next few years.
More info on the 2017 eclipse can be found at http://www.eclipse2017.org
or in the paper itself.
Hugh S. Hudson, Scott W. McIntosh, Shadia R. Habbal, Jay M. Pasachoff, & Laura Peticolas (2011). The U.S. Eclipse Megamovie in 2017: a white paper on a unique outreach event arXiv arXiv: 1108.3486v1
Spicules shooting up from the Sun as seen by the Solar Dynamics Observatory in April. The full disk image is also worth a look. Image credit: NASA/SDO/AIA
One of the many mysteries about our Sun is how its outer atmosphere (corona) gets heated to more than 20 times its surface temperature. Well, it looks like new observations of waving solar seaweed by the sensitive Solar Dynamics Observatory (SDO) might hold the answer.
Of course by ‘seaweed’ I don’t mean the stuff you get here on Earth. No, I’m talking about spicules – giant fountains of plasma which can reach 32,000 miles high (they also look lovely when imaged by the SDO as you can see above). They sway back and forth in the solar atmosphere thanks to ripples in the Sun’s magnetic field called Alfven waves. NASA’s released a neat movie showing them in action.
These waves were thought to be responsible for carrying energy up into the corona, and thus providing the heat needed to get it to the extreme temperatures seen. However, when they were directly observed for the first time in 2007 they looked to be too weak to accomplish this adequately. Luckily for theorists, more recent observations by the SDO have found much more powerful waves, meaning that they can transport enough energy after all.
This is just a quick overview of the story. If you want more information check out the NASA press release or the longer version I wrote for Astronomy Now Online. I wanted to write something about it here too to give me an excuse to post the beautiful SDO image at the top!
McIntosh SW, De Pontieu B, Carlsson M, Hansteen V, Boerner P, & Goossens M (2011). Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind. Nature, 475 (7357), 477-80 PMID: 21796206
De Pontieu B, McIntosh SW, Carlsson M, Hansteen VH, Tarbell TD, Schrijver CJ, Title AM, Shine RA, Tsuneta S, Katsukawa Y, Ichimoto K, Suematsu Y, Shimizu T, & Nagata S (2007). Chromospheric alfvenic waves strong enough to power the solar wind. Science (New York, N.Y.), 318 (5856), 1574-7 PMID: 18063784