Emerald Observatory for iPad |
Emerald Observatory's time may not exactly match the time in the iPad's status bar becausethe iPad's clock is often not very accurate whereas Emerald Observatory's time is synchronized with the international standard atomic clocks. This is usually accurate to about +/- 0.100 seconds. This is accomplished with the Network Time Protocol (NTP) 
so it must have a Net connection to get a sync. If the Net is not available, it will fall back to the internal clock's value.
It is also possible to change the clock's time and to animate it at very high rates. See Set Mode, below.
" 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. (EO does not show the Moon's libration.)
Animate the clock at one day per tick to see the Moon go thru its phases and size changes. Tap the phase button to see the times and dates of the quarters (New, 1st Quarter, Full, 3rd Quarter).
) 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 vegatation 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 Emerald Observatory for its astronomical calculations (usually the current location but you can change it; see Settings, below). It is important that this be the location for which you want the astronomical information to be shown and that the matching timezone is set in the iPad's Settings app.
Animate at one hour per tick to see the terminator (the "edge of night") move around the world. Animate at one month per tick to see the seasonal changes; note the changing shape and position of the terminator.
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.
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The dark yellow ones mark the time when the Sun is 15 degrees above the horizon, indicating the "golden hour" (which maybe 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 on the settings page and then animating by days or months.
Note that the constellation positions shown in Emerald 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 Emerald 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).
("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.
(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.
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.
" 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.
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). The icons are positioned around the ring according to their geocentric ecliptic longitude. 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).
Since eclipses happen at (or very near) full moon and new moon, you can use the "phase" button in Set mode 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 ecilpse 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 Emerald Observatory too, by entering the "location of maximum eclipse" into the latitude and longitude fields on the Settings page, and viewing the eclipse as it will appear there.
Emerald Observatory's database of locations for the Sun and Moon extend back to 4000 BCE, as mentioned above, 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.
stops the clock and enters Set mode. "Set" then changes to "Reset" and two sets of buttons labeled with time units appear on either side:
The blue ones on the right move Emerald Observatory's time forward by one unit; the red ones on the left move it in reverse. The "phase" buttons change to the time of the next or previous quarter Moon (you can then read the time and date from the main display). Tapping Reset changes it back to "Set", exits Set Mode, restarts the clock, and hides the other buttons.
When the displayed time is not the current time a red label appears at the top of the screen showing the displayed time and its offset from the current time.
Emerald Observatory can be set to any date from 4000 BCE through 2800 CE.
The familiar small "i" in the lower right corner
flips the display to a settings and help screen:
The alarm will sound for about 20 seconds (10 rings) or you can tap anywhere on the screen to silence it. It is based on real time only; it does not sound if the alarm time is reached in Set mode. It will sound if the screen is locked while Emerald Observatory is running. If run under iOS 3.2 the alarm will not sound unless Emerald Observatory is the currently active app; with iOS 4.2 or later a local notification will be issued in that case.
The alarm will repeat at the same time each day.
