As a Milky Way or landscape photographer, you quickly realize that the Sun (or Moon or Milky Way) rise time is highly dependent on where you are. You’ll be looking across the landscape wondering why the sun isn’t up yet. You compare the time your app says with your phone. And then you wonder how your app could be so far off?
You already know that latitude, longitude, and elevation are the first considerations for determining when any particular astronomical object will appear on the horizon. But these apps can be off by up to five minutes. Why is that and what can be done about it?
Continue reading and find out the issues involved with calculating rise and set times. Then find out about an new Citizen Science app that may help make these apps more accurate.
34′ or not 34′?
A 2018 Doctor of Philosophy in Physics thesis examines this issue. “Evaluating the Effectiveness of Current Atmospheric Refraction Models in Predicting Sunrise and Sunset Times” by Teresa Wilson of Michigan Technological University looks at how these calculations are made. The standard value for atmospheric refraction at the horizon, 34′ (that’s 34 arcseconds), is not always accurate. This value is used in all publicly available sunrise/sunset calculators.
Photo Credit : Brocken Inaglory (https://commons.wikimedia.org/wiki/File:Green_flash_121007.JPG), „Green flash 121007“, https://creativecommons.org/licenses/by-sa/3.0/legalcode
Wilson found that the standard value results in an uncertainty of of one to five minutes for predictions at mid-latitudes (0-55 degrees North or South). She detemined the error by comparing calculated sunrise/set times with historical observations. These were made at over 30 world-wide locations for 251 sunrises and 514 sunsets. About 600 of these observations had weather data.
Where did the Standard Value Come From?
Wilson determined that the assumptions made by the current atmospheric model uses a correction of 34′ for refraction. But that value fails to take into account actual meteorological conditions. It’s not too surprising as she determined that this correction value can be traced back to at least 1865. It’s possible that the value may have been determined by Sir Isaac Newton!
Sunrise and sunset are defined as the moment the top edge of the Sun (or Moon) is on the horizon. Based on this definition, calculating rise and set times accurate to the second should be possible. But both the U. S. Naval Observatory and the Royal Greenwich Observatory publish astronomical almanacs that only list times for the nearest minute.
Refraction is to Blame!
The problem is the Earth’s atmosphere refracts light that travels through it. If there was no air on the Earth, then the light from astronomical opjects would travel in a straight line. It’s a simple matter of geometry to determine when this happens.
Original: Francisco Javier Blanco González Vector: Jona, Atmospheric refraction, CC BY-SA 4.0
When these objects are near the horizon there are hundreds of miles of atmosphere between us and the object. The thickness of the atmosphere along with the curvature of the Earth combine to refract the light like a gigantic lens. The atmosphere is denser closer to the surface of the Earth than it is higher up. So the light bends progressively downward towards the Earth as it travels towards us.
While atmospheric lensing happens when looking in any direction, the effect is greatest when looking horizontally and when at sea level since there is more air to bend the light. It has less effect as we gain elevation. Factors that affect the refraction include air temperature, temperature gradient through the air column, air pressure, and humidity.
Predicted vs. Observed
Wilson found a seasonal trend in the observed times compared to the predicted times. The predicted times for sunrises that happen over land tended to be early in summer and late in the winter. There was more variation the the summer observations than in winter. This greater variation is probably due more variation in the temperature of the atmosphere during the warmer summer months than winter months.
Wilson points out that, “[…] the atmosphere is assumed to behave the same in Barrow, AK in January as it does in Honolulu, HI in July.”
Mirage effects due to cold air being trapped over warm air that rests over bodies of water had the greatest effect. These conditions could cause sunsets to be as much as five minutes different than the predicted time.
By Land or by Sea
Over an ocean horizon at high elevation, Wilson found the opposite of what is predicted on land, that the refraction is generally much greater than predicted.
Interestingly, Wilson found that new models that attempted to compensate for the meteorological conditions between the observing station and the Sun were not successful. These new prediction models couldn’t make predictions that were any better than 2 minutes from the observed sunrise/sunset.
There’s just too many variables in the weather of the Earth to make better predictions. Utimately, their improved preditions tended to center around the 34′ standard value!
Be a Citizen Scientist!
Wilson concluded that a much larger dataset would be needed to make an improved meteorological model. To collect this data, Wilson developed a free mobile app to do this! It’s called, “Sunrise Sunset Observer” and is currently only available for Android.
This app ties into a Citizen Science program that allows anyone using the app to collect sunrise/sunset data and contribute it to anyone that want to use this data to make a better model.
The app uses your phone’s camera to record a video of the sunrise or sunset. The video is then analysed along with the time that the video was taken to determine exactly when sunrise occurs.
Considerations When Using the App
To use the app, the horizon should be as cloud free as possible. The horizon should have as direct of a line of sight with not too many houses or trees as possible.
As mentioned above, sunrise occurs when the edge of the Sun’s disk just peeks above the horizon. Likewise, sunset is when the disk just disappears below the horizon. Both of these times can be hard to determine visually, so they recommend recording for 15-30 extra seconds both before and after the sun touches the horizon.
They suggest keeping the video shorter than five minutes. Also do not change zoom settings while recording as it makes the analysis much more difficult. You’ll also need to estimate how high the phone is from the ground, including how high you are if you are in a building.
And remember, don’t look directly into the sun!
To read Ms. Wilson’s summary and a link to her entire thesis paper, see here: Evaluating the Effectiveness of Current Atmospheric Refraction Models in Predicting Sunrise and Sunset Times”
Affects on Shooting the Sun (and Moon)
Well, you’re probably thinking, you’ve never had a shot where two or even five minutes of error in the predicted sunrise made a difference. Well, I know I’ve missed sunrises by minutes on several occations! Knowing now that there is some error in the calculations will encourage me to get out there even earlier.
It will also affect exactly where the sun or moon will appear on the horizon. In winter when the sun’s path has a shallow angle to the horizon, a couple minutes of error in the rise time will move the position it appears on the horizon east or west slightly.
For the Moon, this will effect where it will be seen on the horizon more in the summer than winter. Again, this will not have a huge effect on most landscape photographs.
But it will if you are trying to get the use a super telephoto or telescope to get a shot of the Sun or Moon rising from below the horizon. Especially when zoomed in while shooting a time lapse! You don’t want to be making adjustments after the Sun has risen and it’s in the wrong part of your frame.
How Does This Affect the Milky Way?
What does this mean for us as Milky Way photographers? Well, first it means that the Milky Way is effected by the Earth’s atmosphere exactly the same as the sun. So you can’t expect its rise and set times to be any more accurate than those made for the sun.
So the date for the appearance of the Milky Way Core will be effected. But you know what? We shouldn’t really worry about what “exact” date or time the Milky Way Core reappears or disappears below the horizon. Any image with the Milky Way is going to look good! It’s more important to just be out there shooting the night sky!
You can see my photos at www.keyesphoto.com
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