DarkSky Oregon
Welcome to our Blog. Do you want to learn more about how you can help protect the night?
To learn more :
Part 1: Where in Oregon is the Night Sky Most Pristine? And, Most Light Polluted?
DarkSky Oregon and volunteers support a network of Sky Quality Meters across the state which continuously record how bright and how dark the night sky is overhead. The data tell us which areas still have pristine night skies, which areas are polluted by artificial light, and which fall in-between.
This map of Oregon shows the locations of our Sky Quality Meters. The size of the yellow plus symbol tells how starry the night sky is at each spot. The largest plus symbols represent nearly pristine night skies – a very rare environment compared to most of the rest of the USA. The smallest plus symbols represent the light polluted sites – around the cities of Central Oregon, the Willamette Valley and Portland.
The next plot shows in detail how dark the night sky is overhead at our measurement locations – this data was used to make the Oregon map above. The darkest – the most pristine – night sky locations are at the bottom of the plot. The most light polluted are at the top of the plot.
Another way to look at the data in the plot is to compare how much brighter the clear (not cloudy) night sky is at each location by comparison to one of the darker, pristine night sky locations.
The rightmost column of this table shows how much brighter each site is, by comparison to the darkest site measured at present, at Crater Lake National Park. The night skies in the cities of Central Oregon sites are about 5x brighter than at Crater Lake. The night skies in Portland are about 20x brighter.
It turns out that clouds at night reflect light pollution downward in cities, causing the night sky to be very bright. And, away from cities, where there is little light pollution, clouds at night are dark because there is no light pollution to reflect back downward, and moreover, the clouds block out the stars, so the night sky measures very dark.
This last table summarizes the brightness of cloudy night skies at our measurement locations. The rightmost column shows that on cloudy nights, the Central Oregon city skies are 20x brighter and Portland skies are up to 300x brighter than the cloudy night skies at pristine night sky sites, a very un-natural situation.
Support the effort to reduce light pollution. Contact us at:
Key Results from the Oregon SQM Network:
Part 2: Where does the Milky Way stand out best compared to the surrounding night sky?
DarkSky Oregon and volunteers support a network of Sky Quality Meters across the state which continuously record how bright and how dark is the night sky overhead. The data tell us which areas still have pristine night skies, which areas are polluted by artificial light, and which fall in-between. The data also tell us where the Milky Way shines in greatest contrast with the surrounding night sky. We call that an Index of Milky Way Visibility.
This map of Oregon shows the locations of our Sky Quality Meters. The size of the red star symbol tells how well the Milky Way stands out at each spot. The largest stars show the places where the Milky Way stands out best. The smallest star symbols shows where the Milky Way is generally not even visible -- over cities of Central Oregon, cities in the Willamette Valley and Portland.
This map is similar to a map showing where our SQMs measure the most pristine night skies. The plot below shows night sky brightness versus the Index of Milky Way Visibility, demonstrating that similarity, but also allowing separation among the darkest sky sites – on the lower right of the plot.
The plots show that as we would expect, the darker sky sites, those lower on the Y-axis, express increased visibility of the Milky Way. We anticipate that several sites will change position going forward, as additional data, across all seasons becomes available from them all.
The table below shows the data used to make the plot. The Milky Way is most visible at the sites near the bottom of the table, and is mostly not visible for sites at the top of the table.
We calculate the index both from the difference of logarithmic magnitudes/arc second squared data, and from a ratio of linear scale SQM flux data. Both are presented because different observers may prefer one over the other. The indices in each case are consistent, plot against each other in linear fashion (not shown) and each provides useful information.
The first data column in Table 4 lists the difference of the median values at each site, for the night sky brightness when the Milky Way is seen by the SQM versus not. The units are magnitudes per arc second squared.
The second data column in Table 4 shows the same information but as a unitless ratio of linear flux data measured by the SQM. It can be interpreted by noting that, for example, the Milky Way at the Hart Mountain and Crater Lake sites is 1.3x brighter than the surrounding night sky. And at the two Portland sites, the Index value is about 1.0, which indicates that the Milky Way does not stand out in contrast to the surrounding night sky at all.
Support the effort to reduce light pollution.
Contact us at: hello@darkskyoregon.org
Key Results from the Oregon SQM Network:
Part 3: Where are the starry night skies disappearing the fastest in Oregon?
DarkSky Oregon and volunteers support a network of Sky Quality Meters across the state which continuously record how bright and how dark is the night sky overhead. The data tell us which areas still have pristine night skies, which areas are polluted by artificial light, and which fall in-between. Because quite a few of the meters have been operating for two years or more, they also tell us where light pollution has been increasing – where the stars are fast disappearing – and where the night sky has been mostly unchanging.
This map of Oregon shows the locations of our Sky Quality Meters that have been operating for at least two years. Large red diamonds on the map show a strong increase in light pollution overhead, year-by-year. The smallest red diamonds mark sites where no noticeable change overhead has occurred. Most of these sites, all of which have at least 2 years of data, are in Central Oregon because we started this as a pilot project there.
The table summarizes the long-term change information for these sites. The data show increases of 4% to 7% per year for SQM sites near cities. SQM sites away from cities show increases of 2% to 4% per year, while most remote sites tend to show little to no change in skyglow overhead.
It is traditional to quote the percentage change per year, but percentages are comparable to each other only when the starting point is the same. In our case, the starting points all differ – namely we have a range of night sky brightness – from pristine dark to light-polluted bright.
A better gauge is the change over time by comparison to one of the sites where change is slow. So, the first data column in the table shows the rate of change at each site by comparison to the Prineville Reservoir State Park site, which is a certified Dark Sky Park, and where we don’t expect much change in sky brightness. The rate of change at that State Park is given the value of 1.00 and the other sites vary by comparison. Most of the sites in Central Oregon are increasing about 10x faster than at the State Park. The size of the red diamonds in the map are scaled by this rate-of-change column.
As noted, the third column shows the annual percentage change for each site. Note that the percentage change at Prineville Reservoir State Park is estimated to be 1.1% per year, while the percentage change at the Hopservatory is about 6% per year. So, sites with bright, light-polluted, night skies (Hopservatory) appear to have less of an issue when the percentage statistic is used. In reality the rate of change is dramatically larger at the Hopservatory (28x), compared to the State Park site.
Support the effort to reduce light pollution.
Contact us at: hello@darkskyoregon.org
Key Results from the Oregon SQM Network:
Part 4: Where in Oregon are the clearest – least cloudy – night skies?
DarkSky Oregon and volunteers support a network of Sky Quality Meters across the state which continuously record how bright and how dark is the night sky overhead. The data also tell us which areas have the clearest night skies – the least cloudy – and which are most cloudy and in between too.
This map of Oregon shows the locations of our Sky Quality Meters as blue circles. Large circles show the least cloudy measurement sites, which are east and south of the Cascade Mountains. Smaller blue circles mark sites which are more cloudy at night, mostly along the coast and in the Willamette Valley.
The bar chart below shows the data that was used to size the blue circles in the map. Sites on the left side of the bar chart are least cloudy. Those toward the right side of the bar chart are progressively more cloudy at night.
The chart above shows the percentage of SQM data samples that were recorded at each site under clear (not cloudy) conditions, when the sun was at least 18 degrees below the horizon, and the moon at least 10 degrees below the horizon. The SQM sites that recorded the highest percentage of clear night samples are on the left and sorted toward the right to more cloudy conditions.
Also of interest, the Sky Quality Meter data allow a breakdown of the percentage of clear samples by month of the year, as shown in the next chart. Each bar in the chart below shows the percentage of samples on moonless nights by month, which were cloud free.
For those who enjoy a data heavy read, the table below shows the percentage of clear night SQM data samples by month.
The first data column is color-coded, with the SQM sites that recorded the highest percentage of clear night samples at the top, in green color.
The middle data columns show the distribution of clear night samples across months of the year. All of those data columns are color-coded together, so that the green blocks identify the SQM sites and months during which the highest percentage of clear night data was recorded.
The rightmost column shows the total number of clear night samples recorded to date at each site. This column is color coded to show which sites have limited data to date because they are relatively new - the red blocks. Results for the blocks on the red end of the color scale are likely to change as more data is acquired.
White blocks represent no data acquired at those sites during those months so far.
All samples were recorded on a 5-minute cadence. Samples are considered cloud free if the 45-minute period before and after the sample showed minimal variation in brightness. (See Appendix B of the related Technical Report)
Support the effort to reduce light pollution.
Contact us at: hello@darkskyoregon.org
Key Results from the Oregon SQM Network:
