Sky Brightness 3

As we saw in part 1 and part 2 of this series the typical measurements of sky brightness in Providence are between about 4.1 - 4.3 nelm (naked eye limiting magnitude) on clear nights. Here is a graph that shows a typical hazy summer night. The readings were taken on the night of July 1st into the morning of July 2nd of 2014 and are in the range that we commonly see. The dashed horizontal line is a somewhat arbitrary divider between typical and darker nights. When the sky brightness is below about 4.3 the observing is much better.

Looking at a graph of the sky brightness doesn't give an intuitive idea of what the sky actually looked like for observing. We can see this by looking at the wide angle views of the sky using the camera mounted on the roof. Here is a time lapse movie from the same night as the above graph.

Sky Brightness 2

In my previous post I began to analyze the data from the sky brightness meter at Ladd Observatory. Now we'll take a closer look at the broader trends. Here is a scatter plot showing the data from the summer and fall of 2013. The plot is a little busy but we're really only interested in the "bottom line" where the data points are at the lowest values. All of the nights are superimposed on one another with the x axis showing hours UTC. This graph summarizes how the sky brightness changes during the course of the night. The many values between 3.7 and 4.3 are due to nights that are more or less hazy. There moisture in the atmosphere scatters light from the city back down to us and causes the overall sky to look brighter.

If we follow the lowest readings there is a definite trend where the clearest nights start off at about 4.2 at the end of twilight and slowly, steadily, decrease to about 4.45 at 4 hours UTC. There is then a small but rather sudden drop to 4.55 after which the slow decrease continues until we are at about 4.6 in the early morning. I'm not sure what is causing the drop at 4 hours but it may be due to city lights that are on a timer. The takeaway here is that the sky is slightly, but significantly, brighter in the early evening. The best time to observe is after midnight local time through the early morning.

Sky Brightness

"The sky above the port was the color of television, tuned to a dead channel."
- Neuromancer, William Gibson, 1984.

At the Ladd Observatory we operate a weather station and a number of other rooftop instruments to monitor the environment. One of them is a sky brightness meter. On a regular basis we use the live data to judge the quality of the sky for observing. It is also used to document long term changes such as the increase in light pollution.

Sky brightness meter
and camera on the roof.

The meter is contained in a weather proof housing next to a wide field sky camera. The camera takes a low resolution image of nearly the entire sky every 10 seconds and these images can then be compared to the brightness readings. I can then verify what the sky looked like when a measurement was taken. When the sky is very cloudy it scatters light from the city and the readings are very bright. Haze or high humidity can also cause elevated readings.

The sensor is too sensitive to take a measurement during the daytime. It starts collecting data shortly after sunset when the sky begins to darken and stops during morning twilight just before sunrise. Last summer I calibrated the meter and we've now collected 300,000 data points in about one year. I thought this would be a good time to analyze what we have so far.

Geared to the Stars

The telescope at Ladd Observatory uses a clock drive to compensate for the Earth's rotation and track the stars. Modern telescopes use electric motors but this one was built in 1891 before electric power distribution was common. The Observatory originally had gas lamps for lighting and the telegraph system was powered by "gravity cell" batteries. The telescope's mechanical clock drive is weight driven with the speed regulated by a centrifugal friction governor.

A closeup of the clock drive showing the governor in motion.
An optical sensor and precision timer are used to measure the rotation rate.

The Dawn of a New Era

When I was a young child I would watch reruns of the original Star Trek. It wasn't so much the space ships or aliens that impressed me. It was seeing human beings just simply standing on another planet that moved me. It gave me the idea that there were other worlds out there, and that you could travel beyond the Earth to visit them. That sparked my imagination.

My parents would then change the television channel and again I would see people walking on another world. But this time it was on the 6 o'clock news. A grainy video of astronauts in bulky spacesuits standing on a monochrome landscape with the crackling audio of a voice calmly saying "Beautiful, magnificent desolation." It was, arguably, one of the few moments in human history when reality was more amazing than our wildest dreams.

Six months after Apollo 13.
Photo credit: Mom, Halloween 1970.

Prismatic Analysis

One of the projects that I've been working on is the Brashear astronomical spectroscope from 1891. We're trying to recreate the ability to record spectra using the original photographic plate holder. Here is the wooden plate holder mounted in place of the eyepiece assembly that is used for visual spectroscopy.

A focusing screen was created by mounting the ground glass in a foamcore frame that fits the plate holder.

This is Arecibo Calling...

"Ironically, the globular cluster at which the signal was aimed won't be there when the message arrives. It will have moved well out of the way in the normal rotation of the galaxy." - It's the 25th anniversary of Earth's first attempt to phone E.T. Cornell Chronicle, Nov. 12, 1999

My estimate of the Arecibo transmission from a couple of years ago.
It looks like we may have missed...

In 1974 a ceremony was held to dedicate a major upgrade to the radio telescope at Arecibo Observatory. As part of the festivities the telescope was used to transmit a message towards Messier 13, the Great Globular Cluster of stars in the constellation Hercules.

"Scientists Hope to Reach Hypothetical Civilization in a Cluster of Stars" - New York Times, Nov. 20, 1974

Galaxy Collision

Two galaxies colliding and merging.

Generated using the simulation code GADGET-2 running on a small number of nodes on the HPC cluster at the Center for Computation & Visualization and rendered using IFrIT. The T= shows simulation time in billions of years.

The source code computes the gravitational forces between the ordinary matter within the galaxies and the dark matter halos surrounding them. The dark matter is not rendered in this visualization. The code was created in 2000 and last updated in 2005. It is optimized for massively parallel computers with distributed memory.

A somewhat better quality version can be viewed here.

Shatter Cone

Shatter Cone from the Sudbury Basin.

In 1992 I attended a conference on Large Meteorite Impacts and Planetary Evolution in Sudbury, Ontario. The highlight of the visit was the field trip that we took to view the geological evidence of a massive meteorite impact nearly two billion years ago. The impacting object was likely in the range of 10-15 km (about 1/2 - 1 mile) in diameter. It left an oval basin which is 60 by 30 km (40 by 20 miles) containing breccia, a rock formed from the broken fragments of pre-impact bedrock. There are also rocks that have been shattered by the shock wave from the impact. These are only found in impact craters and are called shatter cones.

The Transits of Venus

The 2012 transit of Venus by Michael Umbricht

The June 5, 2012 transit of Venus across the Sun at 6:21:38 PM EDT captured using a modern digital camera through the historic main telescope at Ladd Observatory.

The telescope is a refractor with 12" aperture and 15' focal length. A solar pre-filter was used on the objective of the telescope and the camera used a broadband hydrogen alpha nebular filter to further reduce the brightness due to the sensitivity of the camera. The weather was mostly cloudy during the day and this image was taken during a brief thinning of the clouds which gives the Sun's surface a mottled appearance. 

The photo at left shows Michael Umbricht preparing the telescope and camera before the transit.

The Chemical Furnace

Celestial globe, late 19^th century.

This closeup image of a celestial globe shows an obsolete constellation called Machina Electrica (electrostatic generator) and Fornax Chemica (chemical furnace) which is now simply called Fornax. The latter constellation was originally named le Fourneau by La Caille in 1756 and was Latinized by him to Fornax Chimiæ in 1763. There are a variety of alternate spellings.