User guide

Explore with confidence.

Astra Chron combines a globe, a ground-level observer, and an eclipse simulator. This guide explains both the controls and the astronomical language used by the app.

Getting started

Open the Explorer, enter a place name or coordinates such as 50.7198, -1.1701, then select Go, which places you at ground level at that location in Sky View, with terrain and satellite imagery enabled. The console withdraws as you arrive so nothing covers the sky; bring it back with Location Tools in the sidebar. Use My Location asks your device for your current position. Save current stores the selected location only in this browser.

Use Globe in the sidebar for the worldwide view and Sky View to stand at the selected location. Now sits beside the date and time field at the foot of the screen and returns the clock to the current instant.

Azimuth, altitude, and pitch

Azimuth: The compass bearing around the horizon: 0° is north, 90° east, 180° south, and 270° west.
Altitude: The angular height of a celestial object. 0° is the horizon, +90° is directly overhead, and a negative value is below the horizon.
Pitch: Where the camera itself is looking vertically. Pitch changes your view but does not change the calculated altitude of the Sun or Moon.
Heading: The camera’s current azimuth. The compass shows the same direction using N, NE, E, SE, S, SW, W, and NW.

Twilight and paths

The coloured twilight boundaries show where the Sun has reached specific angles below the local horizon. They are geographic boundaries across Earth, not a weather forecast.

Sunset / sunrise: The Sun’s upper edge is at the apparent horizon.
Civil twilight: The Sun is between the horizon and 6° below it. Outdoor detail is usually still visible.
Nautical twilight: The Sun is 6° to 12° below the horizon. The horizon may remain distinguishable at sea.
Astronomical twilight: The Sun is 12° to 18° below the horizon. Beyond 18°, the sky is conventionally considered fully dark for astronomy.

The Sun and Moon ground tracks connect the geographic points where each body is directly overhead during the UTC day. They do not show the arc that the body follows through one observer’s sky. In Sky View, the body’s changing azimuth and altitude describe that local sky path.

Photo 3D does not show day and night. The graded daylight and darkness across the globe is produced by Cesium’s globe lighting. Photo 3D replaces the globe with Google Photorealistic 3D Tiles, whose aerial imagery has daylight baked into it, so no terminator or night side is drawn while Photo 3D is active. The twilight boundary lines described above are calculated separately and remain both accurate and visible. For a shaded globe, turn Photo 3D off.

Sky View

Look around lets you drag to change heading and pitch. The compass and Current view readout sit together in one card in the lower right, above the playback bar, and stay visible while the panels are open. The Sun and Moon are drawn on the compass rose at their true bearings, with height shown by distance from the centre: the hub is directly overhead and the dotted ring is the horizon. A body below the horizon is dimmed and sits outside that ring. Select the Sun or the Moon on the rose to centre the view on it. Relocate returns to a map so you can click a new observer position.

Moving around on desktop

Drag anywhere to look. Use WASD or the arrow keys to move across the ground, including diagonals. E or Page Up rises and Q or Page Down descends, as far as the 2 m ground floor. Hold Shift with any of these keys to travel three times as far per step. Every step scales with your height, so movement stays fine at ground level and covers distance quickly from altitude. The mouse wheel moves forward or backward along the current heading. The height bar described below is on this layout too, beside the compass, for when you would rather not reach for the keys.

The height bar

The narrow bar down the right-hand edge of Sky View sets your height above the surface. It is the only way to climb on a touch device, which has no E or Q key, and it is on the desktop layout as well.

Drag the handle to travel anywhere between ground level and 1 km. Its scale is logarithmic, so the lower part of the bar gives fine control over the first few metres while the upper part covers the last few hundred; your current height is shown above it. The ▲ and ▼ buttons move one step at a time — a few metres near the ground, a tenth of your height further up, the same notch a mouse wheel or a held E or Q key covers. Hold either button to keep climbing or descending.

On a phone or tablet, the chevron at the foot of the bar folds it away and brings it back, in the same way the sidebar collapses. The bar also stands aside on its own while the Observer Console or Toolbar is open, because those panels use the same width.

Moving around on a phone or tablet

Drag the sky to look around, and spread or pinch two fingers to move forward or backward along the current heading. Height comes from the bar above.

The height presets in the Observer Console are measured above the active surface: Ground is 2 m above it, while 1 km, 10 km, and 100 km provide progressively wider views.

Enhanced keeps the Moon at its real sky position and makes it easier to see. As it approaches the visually enlarged Sun, the Moon shrinks smoothly; a close pair switches to true scale so the display cannot imply a false overlap. True scale always uses unmodified apparent angular sizes. Star and planet controls change which catalogue objects and labels are drawn; they do not alter the astronomical calculations.

Moon Explorer

The dedicated Moon Explorer streams Cesium Moon 3D Tiles only after you open that route. The whole Moon rotates slowly until your first interaction. Drag to orbit, pinch or use the wheel to zoom, and use a two-finger drag to tilt on touch devices. View whole Moon returns to a fully framed global view.

Fly over terrain enters a flight view above the point in the centre of the Moon. On desktop, move the mouse to look around, use WASD to fly parallel to the surface, E to rise, and Q to descend. Hold Shift to boost and press Escape to leave flight. On mobile, swipe the terrain to look and hold the direction, Rise, or Lower controls to move. Movement speed scales with height and the camera maintains clearance above the streamed terrain.

Use Major names for seas and large landmarks, enable Detailed features to reveal smaller IAU names as you zoom, and select an Apollo site to fly to its landing region. Selecting a marker or feature name shows its coordinates and authoritative NASA or USGS record.

The lunar terrain combines NASA Lunar Reconnaissance Orbiter and USGS Astrogeology data. It requires a Cesium ion token with asset 2684829 in its allowed asset list; the required data credits remain visible inside the viewer.

Eclipse Lab

Eclipse Lab compares the true apparent discs of the Sun and Moon from the selected observer. The observer display uses a celestial-north-up frame so the Moon’s passage remains easy to compare at every location without introducing horizon-based field rotation. The globe can show the live penumbra, central shadow, elapsed totality corridor, and upcoming track.

The Next Eclipse bar

Eclipse Lab replaces the scene bar with the Next Eclipse bar across the top of the view, so the eclipse itself—not the terrain controls—holds that position. It carries the kind of eclipse, its date and local time, the obscuration, the altitude of the Sun at maximum, and the C1 to C4 contact times with the length of totality or annularity. Previous, View peak, and Next move between local solar eclipses. The control at the right of the bar folds out the local time zone and the next total eclipse anywhere on Earth; its Previous and Next move the observer and time to the centre of totality, then open the globe so the shadow path is visible. On narrower displays the bar keeps the headline and the date on show and folds everything else behind that one control, so it stays a single strip above the panels. Scene modes are held at their current setting while Eclipse Lab is open.

Centre separation is the angular distance between the Sun and Moon. Obscuration is the percentage of the Sun’s visible area covered by the Moon. Never look directly at the Sun without suitable certified eye protection.

Sidebar, panels, and playback

The Explorer sidebar has three reveal stages. The smallest stage is a single chevron button. Select it once to reveal the icon rail. Every icon is immediately usable: Globe and Sky View switch between the worldwide and ground-level views, Eclipse Lab opens and leaves the eclipse simulator, Location Tools toggles the Observer Console, Viewer Options toggles the main toolbar, Share opens the sharing panel, Moon Explorer opens the lunar viewer, and About opens application and attribution information. Select an empty part of the icon rail—not an icon button—to reveal the text labels and user placeholder. The bottom chevron moves back through the reveal stages.

The Console and Toolbar are independent. Opening one does not close the other, and selecting its sidebar icon again hides it. When both are open, the Observer Console takes the first position and the Toolbar moves to the next aligned or stacked position. Panels animate from the left without resizing as the sidebar changes. On wider displays, the pin control in the fully revealed sidebar keeps that labeled stage open; compact displays retain the icon rail while panels are using the available width.

On desktop, drag a panel by its dotted header. Use the minus button to collapse its contents, the plus button to expand them, and Reset to restore the default arrangement. Double-clicking a drag header also resets the panel layout. With keyboard focus on a drag header, use the arrow keys to move it; hold Shift for larger steps. A panel you move keeps that offset on this device.

The playback bar is always available at the foot of the Explorer, above the site footer. It contains Play/Pause, a time-of-day slider, the local date and time, and a speed selector. Negative speeds run backward. The date and time field and its Now button share that foot of the screen: alongside the playback bar on wide displays, and on their own row directly above it on compact screens. The playback bar is fixed in place and is not one of the draggable panels.