What is sunrise?

Stephen Trainor
Dec 22, 2016 · 6 min read

From time to time we’re contacted by users who believe that The Photographer’s Ephemeris has given incorrect times or directions for sunrise, sunset or moonrise, moonset, and so forth. (Any reference to sunrise in this post generally applies to rise and set events for any object, including the sun, moon, and Milky Way.)

We take all such reports seriously and are happy to investigate them.

This post covers off a few points of interpretation that occasionally lead users to believe that the app is giving incorrect information.

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 — for example, my complete lack of awareness of the subtleties of spherical trigonometry which led to the azimuth lines being mis-placed, or my failure to bound right ascension within 360°, which led to moonrise being given for an off-by-one date. Those bugs were reported by users and we were able to fix them quickly.

Since that time, the code that calculates sun, moon, Milky Way and star positions hasn’t really changed that much. Nowadays, we have tens of thousands of monthly active users, so 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.

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 working 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 true horizon — that is, specifically not the visible horizon.

If you don’t have a clear view to the horizon, then you can’t see the moment of sunrise or sunset

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).

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 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.

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 it is 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:

Sunrise from Mauna Kea, with elevation at the horizon set to zero.

If after considering the above information you still believe you’re getting bad results, then check out the additional instructions here and send us your data. It’s vitally important to us that TPE’s results are dependable, so we’ll be sure to investigate thoroughly.

So why can’t TPE calculate ‘effective’ sunrise?

Reasonable question. However, it’s probably first worth asking what is the value of knowing the time and direction of official sunrise?

A simple thought experiment serves to illustrate: imagine you’re standing at the foot of a high sea cliff somewhere on the west coast. Imagine that it will take three hours or more for the sun to become visible above the cliffs behind you.

From the perspective of a photographer, you really want to know the time and direction of true sunrise: that’s when the sky opposite and above may light up with the best color and when the shoreline in front of you will be in soft (albeit indirect) light.

By the time the sun actually clears the cliff, the skies will be bright and the light harsh and relatively cool (that is, a higher colour temperature).

However, it’s fair to say that many photographers would want to know the time and direction of effective sunrise — or, more likely, full moonrise.

If you want to the catch the full moon appearing above a well-known mountain summit, this is the information you need. You can use the geodetics tools in TPE to determine this manually, or Visual Search in TPE for iOS to find times when this alignment will occur:

However, to determine the ‘effective’ rise or set time/direction automatically requires a detailed model of the 3D topography around the red pin location and a trial and error approach to calculation. You can make no assumptions about the surrounding terrain: in some places, the sightline will fall away from an obstruction, in others it will rise:

Where will the sun actually first appear?

That’s a lot of data and expensive calculations for something that most users don’t need, so we don’t attempt it in TPE right now. However, as Scarlett said, tomorrow is another day, so watch this space for more news.

Update Jan 2019: in reposting this story today, I realised the teaser about a potential solution for ‘effective sunrise’ was still there. Of course, since writing this post, we released TPE 3D which gives you a direct view of exactly when the light will reach different parts of the landscape. You can find out more about it on the App Store.

The Photographer’s Ephemeris

The Photographer’s Ephemeris (TPE) is a tool to help you…

The Photographer’s Ephemeris

The Photographer’s Ephemeris (TPE) is a tool to help you plan outdoor photography in natural light, especially landscape and urban scenes. It is a map-centric sun and moon calculator: see how the light will fall on the land, day or night, for any location on earth.

Stephen Trainor

Written by

Software, photography, art, and music. Maker of @photoephemeris.

The Photographer’s Ephemeris

The Photographer’s Ephemeris (TPE) is a tool to help you plan outdoor photography in natural light, especially landscape and urban scenes. It is a map-centric sun and moon calculator: see how the light will fall on the land, day or night, for any location on earth.

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