From time to time we're contacted by users who believe that TPE is giving incorrect times or directions for sunrise, sunset or moonrise, moonset (etc.)
We take all such reports seriously and are happy to investigate them.
However, there are a few things that might lead you to believe the app is incorrect. This post covers off a few common points of confusion and sets out what we need in order to investigate your report.
TPE was first released in early 2009. Back in those first few months of beta testing we did indeed have a couple of bugs that resulted in the app giving incorrect information in certain conditions. Those bugs were found by users like you and we were able to fix them quickly. However, since that time, the code that calculates sun, moon, Milky Way and star positions hasn't really changed that much and we have tens of thousands of monthly active users. We're pretty confident that any fundamental issues in the calculations would be found out very quickly and reported by a large number of users.
Please review the information below and send us the details requested at the end of post if you still have a concern about TPE's results.
Checking the basics
While it seems obvious, it's vital to confirm that the app is configured correctly:
- Is the red pin set at the location you need? Calculations are performed for the red pin location.
- Do you have the correct date set in the app? Being off by one day makes a significant difference for the moon, in particular.
- Is the indicated time zone correct? The app determines time zone automatically, but needs a network connection to update this. Also, some locations near time zone boundaries may use a non-standard time offset, so it's always worth double checking.
- If the numerical azimuths seem incorrect in TPE for iOS, do you have Magnetic North enabled in Settings? Are you using a field compass? Is it offset for magnetic declination or not? Both app and compass need to be used consistently either for true north or magnetic north.
What is sunrise?
One common misconception arises from the definition of rise and set. Sunrise is defined as the moment when the "upper limb" of the sun first appears above the horizon.
If you don't have a clear view to the horizon, then you can't see the moment of sunrise (or sunset). You can only see to the horizon in flat terrain, clear of obstacles (for example, looking out to sea).
This is not sunrise:
This isn't sunrise either:
This IS sunrise:
If you can't see the sun until it has risen above an obstacle, such as a mountain or hill, then you will only see the sun after the standard time of sunrise and the direction will not be the same. In the northern hemisphere, the sun will track to the south after sunrise and vice versa in the southern hemisphere. In the tropics, the direction will depend on the exact location, time of year and time since sunrise.
So, first, make sure that you're actually observing true sunrise etc. and not simply the moment when the sun appears above an obstacle. If it's the latter, then you can use the Geodetics tools in TPE to check sightlines to mountains, ridges etc. to determine when the sun will appear - see our Tutorials page for more information.
If you can't see to the horizon, then the sun or moon will appear after rise and/or disappear before set.
But sunrise is happening earlier than the app says...
If you can see to the horizon and the sun appears earlier than advertised, then it's likely that you're observing sunrise from a location elevated above the surrounding terrain.
Just as you can see farther from the window of an airplane, the same applies on mountain tops, or high clifftops overlooking the sea. In these circumstances, the distance to the horizon is greater than for an observer on the ground, and therefore you will observe sunrise earlier:
And because you see the sun earlier, its position on the horizon is at a different bearing (farther north in the northern hemisphere, farther south in the southern hemisphere in general).
Rise and set times are calculated by default without adjusting for the observer being raised above the local horizon. However, in TPE you can include this adjustment by setting the elevation at the horizon. See our Tutorials for more details.
From some locations (e.g. the summit of Mauna Kea in Hawaii), height above the horizon can result in sunrise being observed up to 10 minutes earlier than the standard time.
It's still wrong
If you believe that the app is still giving incorrect information (and you've double checked the obvious, i.e. the red pin is set at the location you need and you have the correct date selected), then it's time to check the data you're seeing against some alternative sources.
Three free resources on the web that you can consult are:
Please check your date and location in at least two of these tools and if you see any material disagreement with TPE (i.e. times differ by more than 1 minute, or azimuths differ by more than half a degree), send us full details of your example, with screenshots and we can investigate further (see below for details).
The numbers agree, but the direction on the map is wrong
If the values in TPE agree with the other sources, but you think the direction shown on the map is incorrect based on your observations at the location, then please double check the bearing you're looking at.
We've had a number of cases where users believed the line direction shown in TPE was incorrect, but on careful checking with a field compass (appropriately adjusted for local magnetic declination), the direction they thought they were looking in was in fact different than assumed.
You can also check the angle drawn on the map in TPE with a protractor, or indeed check TPE's indicated direction against an alternative app that shows the same information.
If you think the direction indicated on the map is incorrect, please provide details of your own observations and measurement of direction of sunrise etc. in the field.
What we need to investigate further
If you'd like us to investigate, here's what we need:
- Latitude, longitude of the location
- Expected time and azimuth or rise/set
- Actual observer time and azimuth of rise/set
- Screenshots from TPE
- Screenshots from two of the alternative sources listed above showing results for the same location/date
- Details on measurement of direction of sunrise (etc.) in the field, if that is the issue
- A photograph of the rise/set event is often very helpful also, ideally with an accurate timestamp, and if possible, geotagged with GPS coordinates (your phone will do this automatically if Location Services are enabled for your camera app)
It's vitally important to us that TPE's results are dependable, so we'll be sure to investigate thoroughly!