This tutorial applies to The Photographer’s Ephemeris Web.
The 3D Sphere also includes maps and 3D terrain – TPE Web Tutorial, Part 9 describes this functionality. But you should read this, Part 7, first!
The “ 3D Celestial Sphere ” is a reinterpretation of the Armillary Sphere that was invented by the Greeks and has been in use for over two thousand years. It is a simple way to get a sense of the position and orientation of objects in the sky – particularly, the night sky.
If you’re interested in the history and use of armillary spheres, both Wikipedia and YouTube have some good resources.
Viewing the Sphere
Compatibility: your browser and computer must support both WebGL and ResizeObserver to use this functionality. Both are widely supported today, but older browsers may not work.
You can access the 3D Celestial Sphere by clicking on the 3D link in the navigation bar:
What can I use it for?
The 3D sphere is perhaps most useful for visualising the orientation of the band of the Milky Way and Galactic Centre. It can also be used to visualise relative positions of the Sun, Moon, and meteor shower radiant points. You can do all this relative to the secondary Geodetics pin, making it easy to see if the Moon is below the mountain summit or not.
Let’s delve into the details.
Basics
If you switch off the Sun, Moon, Galactic Centre, and Meteors in Settings, you can see a bare-bones sphere, as shown below:
The sphere always displays data for the primary (red) map pin location. The pin is shown at the centre of the ground plane disk. Azimuth and altitude are indicated by the labels and helper lines. Altitudes are marked for +6°, +0°, -6°, -12°, and -18°, corresponding to “ golden hour ” (whatever that is 😉), rise/set (approximately), and civil, nautical, and astronomical twilights respectively.
The Sun
With the Sun re-enabled in Settings, let’s look at how the path and position are shown:
As you adjust the time of day slider or date, you will observe the Sun moving to its correct position. When adjusting the time of day, the Sun will move along the indicated path. On changing the date, the path itself is recalculated.
The azimuth indicator – the orange triangle pointing to around 100° from north in the screenshot above – indicates the direction to the Sun from the primary pin. When the Sun is very high in the sky, such as in the tropics, the azimuth can be hard to perceive visually. If you’ve ever been confused as to why the azimuth changes so rapidly as time of day is adjusted in places such as Timbuktu, the 3D Celestial Sphere can make it clear what’s actually going on.
The primary map pin is modelled as a 3D object and will cast a shadow from the virtual sunlight. Nothing in this model is to real-world scale; therefore, absolute shadow lengths cannot be determined from the pin. However, relative shadow lengths quickly become apparent.
The sunlight is adjusted for both colour temperature and intensity based on altitude – you’ll see the light turn redder towards sunset.
Important note: the Sun and Moon are displayed at an exaggerated scale. They would be hard to see at their true apparent size of ~0.5°. Therefore, at rise and set times, the body of the Sun (or Moon) will still be visible above the ground plane. However, rise and set remain defined as being the time when the upper limb of the body becomes visible or disappears from the apparent (unobstructed) horizon. See What is sunrise? for more details on the definitions.
Adjusting the view
You can adjust the camera position freely to get a better sense of how things line up. Here’s another view of the Sun:
You can ‘orbit’ the red map pin using the mouse or trackpad. Here, the view is rotated to look from around 225° and a lower altitude, and the camera has been zoomed in.
Here are the various gestures you can try out:
- Rotate around centre point: Left-click + drag
- Fly in/out: Trackpad pinch gesture / mouse scroll wheel
- Pan: Trackpad Shift + left-click + drag / mouse right-click + drag
Note: If you find the whole browser page is zoomed in/out when trying the trackpad pinch gesture, instead try dragging up or down with two fingers. This may be necessary in Safari on macOS.
The Moon
When you add the Moon back in Settings, it appears in the familiar blue colour shown on the map page:
Note: viewing the Moon, Galactic Centre, Milky Way, and meteor showers in the 3D Sphere requires a PRO subscription.
The Moon is shown similarly to the Sun, with current position, path for the selected 24-hour period, azimuth indicator, and a pointer originating from the primary pin.
Note that there is a break in the path: this is because over a 24-hour period, the Moon does not return to the same position, as expected given its 29.53-day cycle. (You can see the same discontinuity for the Sun, particularly around the equinoxes when the rate of change of the position of the Sun is at its greatest.)
The phase of the Moon is not indicated in the 3D representation, but you can check the illuminated fraction in the timeline below. The Moon shines its own light and will cast shadows as with sunlight – you’ll only see them when the Sun has set. The intensity of the moonlight is adjusted for phase.
Milky Way and the Galactic Centre
Finally, upon enabling the Galactic Centre in Settings, we can see the band of the Milky Way and related information:
The Solar System lies in the arm of a spiral galaxy, the Milky Way. As a result, it appears from Earth that the stars of the Milky Way surround us in a ring. However, we’re far from the centre of the galaxy, and so there is a higher density of stars and interstellar dust towards the galactic centre. This is typically the most photogenic part of the Milky Way, and so photographers find it useful to know its location in the sky.
The path of the Galactic Centre alone is typically insufficient to plan a Milky Way photograph. In addition to knowing where the centre lies, it is often preferable to know the orientation of the band of the Milky Way. This is represented as a circle of spheres. The larger, brighter spheres occur towards the galactic centre, with the centre itself being the largest and brightest.
For wide-angle night photography, the band of the Milky Way can serve as a framing device for a building, structure, or landscape feature, where the stars appear to arch over the top of the subject. To frame your subject this way, you need to shoot ‘through’ the arch. For this reason, an additional azimuth indicator is included showing the direction towards the highest point in the band of the Milky Way, shown as the “ top ” of the Milky Way in the screenshot above.
Standard Mill Under Milky Way Arch, 13 July 2020, © Jeff Sullivan, courtesy of www.JeffSullivanPhotography.com
At low latitudes in the northern and southern hemispheres, the Galactic Centre can be the highest point above the horizon in the band of the Milky Way. In the following example, for a location in Namibia in August, the two azimuth indicators almost coincide (and would overlap with a few minutes’ time adjustment).
Note that the Galactic Centre is not visible at northerly latitudes. For example, even in the height of summer, it transits below the horizon in Reykjavik, Iceland (link):
Of course, even if it did rise, the sky would be too bright to see it, as the Sun has only just set.
Meteor Showers
When enabled in Settings, the 3D Celestial Sphere will show the apparent radiant point for major meteor showers, for example, the Perseids, shown here just after moonrise, shortly after midnight on 12 Aug at Stonehenge (link):
Each meteor shower is shown during its active dates. The name and peak active date are displayed next to the icon:
You can click the link icon next to the peak date of any meteor shower to view further information (typically, this is a link to the corresponding Wikipedia page).
If you are planning to spend a night photographing meteors, it is typical to make multiple exposures over several hours, find the frames containing meteors and then combine the images. Collectively, the meteors will typically appear to be emerging from the radiant point. It can therefore be useful to ensure you have included the radiant point in your composition, bearing in mind that for some meteor showers and locations, the radiant will move significantly in the sky across the course of several hours.
For example, while the Perseids’ radiant point remains relatively static in the north-east sky in the hours after astronomical twilight ends, the Geminids, in contrast, move significantly further over the same time period.
What lies beneath
By default, the sphere cuts off at -18°, which is the limit of astronomical twilight. After the end of astronomical twilight the sky is truly dark. Any remaining light will likely be due to artificial light pollution or airglow. Once the Sun is below -18°, you are generally no longer concerned for night photography.
However, if you wish to see beyond the -18° altitude limit, simply rotate the view lower, and the full sphere is revealed:
Use with Geodetics
If you are unfamiliar with geodetics, please refer to TPE Web Tutorial, Part 3: Geodetics.
When Geodetics is enabled, the secondary grey map pin is also displayed on the sphere. Generally, this is only useful if the elevation angle from primary to secondary pin is positive, e.g., the primary pin is in a valley and the secondary pin is on a hilltop.
Here’s an example from Boulder, Colorado. Let’s imagine we wish to photograph the morning Moon over Green Mountain from Chautauqua Park meadows (link):
The 3D Celestial Sphere can be used to check the relative position of the summit of Green Mountain and the Moon. The Moon is seen to lie higher than the secondary pin:
This can be a convenient way to visually check relative az/alt (azimuth / altitude) positions.
Note: The sizes of the Sun and Moon are exaggerated in the 3D model. Always check the numerical values for high-precision work.
Use with Elevation above the Horizon
If you are unfamiliar with adjustment for elevation above the horizon, please refer to TPE Web Tutorial, Part 4: Horizon.
If adjustments for elevation above the horizon are enabled, the dip of the horizon is reflected in the 3D Celestial Sphere.
For example, when standing on the summit of Everest and looking south to the plains of northern India, there is significant elevation above the horizon:
The dip of the horizon is also visible in the 3D Celestial Sphere:
The apparent horizon is shown at an altitude of -3.29°. The ground plane is displayed as a cone instead of a flat plane. With this adjustment, it becomes much clearer that the timing of rise and set events is altered for the observer standing on top of Everest:
We hope you find the visualisation tools provided by the 3D Celestial Sphere useful in your photo planning. Please send us your feedback and questions.
