A web port of Emerald Observatory (GitHub), an astronomical clock app originally built for iPad.
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Your settings are saved in this browser's local storage on this device. Use the Share button to copy a link to a view. See Privacy for more information.
A web port of Emerald Chronometer (GitHub), an astronomical watch-face app originally built for iPhone and iPad.
A web port of Emerald Observatory (GitHub), an astronomical clock app originally built for iPad.
A live data explorer for the astronomy engine shared by Chronometer and Observatory — rise/set times, planetary positions, and calendar and clock values, at any time and location.
Thirty years of solar and lunar eclipses, each with links that open the moment and place of maximum eclipse in Observatory and Chronometer.
Emerald Observatory for the Web
Emerald Observatory for the Web is a fully client-side application. It runs entirely in your browser and does not communicate with any application server for its core functionality.
The app itself does not collect, store, or transmit any personal data to any application server. There are no app-level analytics, tracking pixels, cookies, or telemetry of any kind.
When the app is served over HTTP or HTTPS, the hosting provider (SiteGround) records standard web server access logs for each page and resource request. These logs include:
These logs are retained by SiteGround for up to 30 days and are used solely for security monitoring and troubleshooting. They are not analyzed for user tracking purposes.
When running from file:// URLs (a local copy of the app), no
server is involved and no logs are generated.
When you use the location dialog (on pages served via http:// or
https://), the app fetches map tile images from
OpenStreetMap
to display a preview of the selected location.
These requests are standard image fetches and do not include any metadata identifying the tile coordinates as your location — OpenStreetMap sees only which map tile was requested, the same as any map browsing.
If you tap "Use device location via browser", the app uses the browser's Geolocation API to obtain your coordinates. This data is processed entirely within the browser — the app never sends it to any application server.
The resolved coordinates are matched against a bundled database of approximately 47,000 cities to display a human-readable location name. This reverse-geolocation lookup happens entirely in the browser — no external geocoding service is contacted.
The app saves your settings (location, time, and per-app configuration) in your browser's local storage — on your device only. This data is never sent to any application server, and it persists across visits without any cookies.
You can clear local storage at any time through your browser's settings (typically alongside your browsing history).
When you use the Share button, the app builds a URL that encodes the current view so you can send it to someone else or open it on another device. Opening such a link lets you either use those settings just for that visit or save them as your defaults.
Note: Because shared links carry settings as URL query
parameters, those parameters are visible in your browser history and, when
served over HTTP or HTTPS, in the hosting provider's access logs — but only for
links you deliberately create or open. On file:// pages where
local storage is unavailable, the app falls back to keeping settings in the
URL.
For questions about this privacy policy, please visit the project on GitHub.
Last updated: May 2026
Emerald Observatory for the Web
Found a bug? Please report it on the GitHub issues page. Describe what you saw and what you expected, and mention which app and which browser and device you were using, along with the app’s version number (shown at the bottom of the help popup opened by the ℹ︎ button, e.g. “v2.1.8”).
This free web app is maintained by volunteers, so no support or response time is promised — but the developers do read the reports and fix most bugs.
The website itself only serves, as a convenience, files available from the GitHub releases page (see instructions for running the app here).
Emerald Observatory for the Web
THIS FREE WEB APP IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
The pages served to your browser are available at the GitHub releases page (see instructions for running the app here).
Clock
First of all, it's an ordinary clock. The main hands (gold colored) display the hours, minutes and seconds in the usual 12-hour format. To read it more precisely use the small numbers and tick marks on the inner edge of the rings. The thin central hand with the large white arrow head and the large white numbers and tick marks indicate the time in 24-hour format (with midnight on top by default; see Noon on Top, below).
It is also possible to change the clock's time and to animate it at very high rates. See Setting the time, below.
Moon
In the upper left Observatory displays the Moon as it appears from the current
location at the clock's time. When the phase is a thin crescent Observatory
simulates the illumination of the dark side by reflected light from the Earth
(this is known as "Earthlight" or "Earthshine
" or "the Moon's ashen glow"). In addition, the image rotates to match the
Moon's apparent orientation as it moves across the sky and changes size to
match the Moon's apparent size as its distance from the Earth changes around
its elliptical orbit. (Observatory does not show the Moon's libration.)
Animate the clock at one day per step to see the Moon go thru its phases and size changes. Choose the time controller's phase unit and step ◀ ▶ to jump to the times and dates of the quarters (New, 1st Quarter, Full, 3rd Quarter).
Earth
The map of the Earth (a
NASA composite of thousands of images) at the top of the screen shows which parts of the Earth are facing the Sun
and which parts are in night. The image changes each month to reflect the
changing snow and vegetation cover. The lights of mankind's cities are shown
for the nighttime region. They roughly correspond to the human population
density and level of energy use at the present time. (The data was collected
by the US Air Force DMSP satellites in the mid 1990s.)
The small red dot on the Earth marks the current location being used by Observatory for its astronomical calculations (usually your current location but you can change it; see Setting the location, below). It is important that this be the location for which you want the astronomical information to be shown.
You can click and drag on the Earth map to explore how the display changes at different locations. While dragging, a red crosshair marks the explored location and the red dot stays at your home location. Hold the Shift key while dragging to constrain movement to latitude only or longitude only (whichever direction you drag more). Hold the Option (Mac) or Alt (PC/Unix) key while dragging to lock the timezone to your current timezone, showing what astronomical events look like at other locations relative to your home time. When you release, a prompt asks whether to keep the new location or revert to the original.
Animate at one hour per step to see the terminator (the "edge of night") move around the world. Animate at one month per step to see the seasonal changes; note the changing shape and position of the terminator.
Day/date
The year, month, day and day of the week appear near the top of the screen, in the calendar of the day: the Gregorian calendar from 15 October 1582 on, the Julian calendar before that. "BCE" follows the year for dates Before the Common Era (a.k.a. "BC"). For dates in the Julian era, up to 4 October 1582, a small "Julian" follows the year; BCE dates are always Julian (proleptic Julian before 1 BCE), so they carry it too. A small "leap" appears in leap years, under the rules of the calendar in force.
The small abbreviation is the location's time zone at the displayed instant. Before standard time zones existed (roughly the 1880s, later in some places) it reads "LMT", local mean time: the time zone database assigns such dates the mean solar time of the zone's reference city, and so does Observatory. With a mouse, resting the pointer on the date spells all of this out, with the zone's exact offset from UTC.
Main dial
Rise/Set rings
There are 6 thin concentric partial rings, one for each of the classical "planets". The innermost one is for the Moon, followed by Mercury, Venus, Mars, Jupiter, and Saturn on the outside. The larger one behind the others is for the Sun. The filled region of the ring represents the time (read against the 24-hour markings) when the planet is above the horizon; it rises and sets at the ends of the arc and transits at the midpoint. Thus if the thin white arm of the 24-hour hand crosses the filled part of a planet's ring then that planet is currently above the horizon.
The large ring for the Sun is colored according to the Sun's altitude above the horizon at each time; sunrise and sunset are at the sharp boundary between red and blue.
Twilight hands
There are 12 colored hands on the 24-hour dial which mark special times
related to the altitude of the Sun. The light yellow one marks the time of
solar noon, when the Sun is at its highest point in the sky for the day; the
dark blue one is 12 hours later when the Sun is farthest below the horizon.
The orange ones mark the times of sunrise and sunset. The three blue ones mark
the times when the Sun is 6, 12, and 18 degrees below the horizon which define
the beginning and ending of civil, nautical and astronomical
twilight. The dark yellow ones mark the time when the Sun is 15 degrees above the
horizon, indicating the "golden hour" (which may be considerably longer than
an hour at high latitudes) when the light is usually best for outdoor
photography.
Note that at high latitudes when the sun's altitude changes slowly the golden and blue regions will elongate. In extreme cases some of the twilight hands will disappear altogether if the Sun never reaches their level. See this effect by changing to a high latitude with Set Location and then animating by days or months.
Timezone indicator
Between the center and the 6 o'clock position is a simple label showing the abbreviation of the current timezone. It changes to reflect daylight time (summer time) vs standard time for those timezones that observe DST. Observatory's astronomical calculations depend on the timezone offset and geographic location; the timezone is determined automatically from the selected location.
Orrery
Inside the main dial Observatory displays the orbital positions of the 6 innermost planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) plus the Earth's Moon from a perspective above the Sun's north pole with respect to the present positions of constellations of the zodiac. The approximate positions of constellations are marked by their traditional astrological symbols in large gray type.
Note that the constellation positions shown in Observatory are based on their actual positions in the present-day sky and thus do not correspond to the definitions used by western astrologers.
The best rate for animating this display
depends on which planet you're concerned with.
Kepler's third law
shows that the inner planets orbit much faster than the outer ones. So
animating by days works best for Mercury or the Moon whereas years is best for
Saturn.
If you animate by years you'll notice that the Earth jitters a little. This is
not a bug. It's because we're advancing by exactly one calendar year but the
Earth's orbital period is about 365.25 days. So the Earth falls behind a
little for three years and then catches up on the leap year. Prior to 15
October 1582 Observatory uses the
Julian calendar
rather than the
Gregorian, so you'll see the Earth jump in 1582 and then it will gradually shift in
position from year to year back from that time (reflecting the error in the
Julian calendar).
Inner subdials
UTC
UTC
("Coordinated Universal Time") is a worldwide time standard based on atomic
clocks that is adjusted (by the occasional addition of "leap seconds") to be very close to the mean solar time at the Royal Observatory at
Greenwich, England ("GMT"). Ordinary civil time in most locations is defined
by an offset from UTC.
Solar
Solar time
(technically: "apparent solar time") is what everyone used before the
invention of clocks and timezones; it will show 12 noon at exactly the time
when the Sun crosses the
meridian
(of course, this is also when the 24-hour hand coincides with the solar noon
hand on the outer dial). Apparent solar time is what an ordinary sundial
shows.
Sidereal
Sidereal time
tracks the apparent motion of the stars around the Earth; the Earth rotates
exactly once in 24 sidereal hours as measured against the stars. That 24-hour
sidereal day is a few minutes shorter than a solar (or civil) day because of
the Earth's motion in its orbit around the Sun. A star that is overhead at a
given sidereal time during the day will always be overhead at that same
sidereal time, no matter what season it is.
The sidereal time dial is labeled with the abbreviations of the zodiac constellations and the corresponding numbers. The hour hand points to the constellation that is currently near the meridian.
UTC and sidereal time are shown in 24-hour format; solar time in 12-hour format.
Outer subdials
Altitude and Azimuth
The two dials on the left side of the screen show the altitude (angle of the object up from the horizon) and azimuth (angle of the object around the horizon clockwise from North) of one of the planets at the clock's time. The body's name sits at the top of each dial between two small ‹ › arrows: tap the left half of either dial (or ‹) to step back through the 7 "planets" (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn) and the right half (or ›) to step forward; the half you tapped lights up briefly, the name slides in the direction you tapped and the hands sweep to the new body.
Equation of Time
The "Equation of Time" is the difference, due primarily to the ellipticity of the Earth's orbit,
between apparent solar time (see above) and mean solar time. Mean solar time
is apparent solar time averaged over a whole year. Ordinary civil time is very
close to mean solar time at the standard meridian of each timezone. So EOT is
the difference between your watch and a sundial on the standard meridian of
your timezone.
The dial is marked in minutes; positive means the sundial is ahead. In the days of celestial navigation, the EOT for the day was an essential input to a navigator's longitude calculation.
Eclipse Simulator
The small dial labeled 'Eclipse Simulator' has three icons in the outer ring,
representing the Sun, Moon, and Earth's shadow and two thin red lines
indicating the nodes of the Moon's orbit (points where it intersects the plane
of the ecliptic). When both the Sun and Moon coincide with a node then an
eclipse is possible. The area inside the ring is normally empty but near the
time of a lunar or solar eclipse an animation of the eclipse will appear
there. Lunar eclipses are visible from an entire hemisphere of the Earth but
solar eclipses are visible only from a very small area. See
here
for a list of times and locations of recent and upcoming eclipses (and much
more). For a table of every eclipse from 2011 through 2041 — with links
that open each one in Observatory — see the
Eclipse Table.
Since eclipses happen at (or very near) full moon and new moon, you can use the Moon Phase steppers in the time controller's Astro tab to move forward to new and full moon dates, and see if there is anything inside the Eclipse Simulator subdial. Eclipses will only happen when the red lines indicating the nodal points are close to the Sun and Moon. You may need to move forward or backward by hours or minutes from the phase point to see the point of maximum eclipse at your location for a given eclipse. Note that solar eclipses, which are much rarer, typically are only visible on a portion of the Earth's surface. But you can simulate those eclipses with Observatory too, by entering the "location of maximum eclipse" as manual coordinates in the location dialog, and viewing the eclipse as it will appear there.
Observatory's database of locations for the Sun and Moon extends back to 4000 BCE, but there is somewhat less precision for those early dates than there is for modern times. Since the difference between total and partial eclipse can be a small fraction of a degree, historical eclipses may not be shown with 100% fidelity. Modern events, however, are shown with very high precision.
Setting the time
Tap "⏱ Show time controller" at the lower left to open the time controller. Choose what a step means — a year, month, day, hour, minute or second, or an event: the rise, set or transit of any body, or the Moon's quarter phase — then tap ◀ or ▶ to step, or hold either to scrub (the panel fades while the scrub runs, and slides off the screen's edge to keep scrubbing hands-free). Below the pair you can type a date and time directly (with a BCE toggle for ancient dates). Escape closes the panel once nothing else is open, and so does a tap on the display.
When the displayed time is not the current time a red label appears in the footer showing the offset from the current time, along with a "Now" button that returns the clock to the present.
Observatory can be set to any date from 4000 BCE through 2800 CE.
Noon on Top
Most 24-hour clocks have midnight (0 or 24) at the top of the dial. But when the clock also shows an indication of the day/night hours it is convenient to have noon (12) on top. As there seems to be little consensus on this issue, Observatory lets you choose for yourself: open Settings (the ⚙ button at the top right, the first row of the ⋮ menu on a phone, or the , key) and, under the Observatory heading, switch Noon at the top of the 24-hour dial. Your choice is saved on this device, so it persists across visits, and it is included when you share a link to Observatory.
Setting the location
Tap "Set Location" at the lower right to choose the location used for the astronomical calculations. You can search for a city or airport code, use your device's location, or enter latitude and longitude manually (in decimal format with negative numbers for west longitudes and south latitudes). Check the red dot on the Earth map to ensure the values are correct. The timezone is set automatically to match the selected location.
You can also drag directly on the Earth map to preview any location — all the dials and rings update in real time as you drag. On release you'll be asked to keep the new location or revert.
