1. CALCULATION OF ASTRONOMICAL DATA
INITIAL FORECASTER COURSE

2. CALCULATION OF ASTRONOMICAL DATA
Knowledge of astronomical information is very important for Aviation.
It helps in:-
Planning of flight
Navigation
Weapon and armament use

3. CONCEPT OF RISING & SETTING
During the course of a day the Earth rotates once on its axis causing the phenomena of rising and setting.
All stars and planets seem to appear in the sky at the horizon to the East of any particular place, then to cross the sky and again disappear at the horizon to the West.
Most noticeable of these events are rising & setting of Sun & Moon.
Sun & Moon appear as circular disks & not as points of light - thus a definition of rise or set must be very specific.
IFC - Astronomical Data

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SUNRISE & SUNSET
Sunrise & sunset refer to times when upper edge of disk of Sun is on the horizon. Atm conditions are assumed to be average, & location is in a level region on Earth's surface.
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the East.
Sunset is defined as the moment the trailing edge of the sun's disk disappears below the horizon in the West.
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DAWN & DUSK
Dawn is time that marks beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon.
Dusk is the moment after sunset at which darkness falls, when the sun is about eighteen degrees below the horizon.
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TWILIGHT
Twilight is the time between dawn and sunrise & sunset and dusk.
Twilight is defined as the period before sunrise and after sunset during which there is natural light provided by upper atmosphere, which receives direct sunlight and reflects part of it toward Earth's surface.
Three types of Twilight – Civil, Nautical & Astronomical.
IFC - Astronomical Data

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TWILIGHT
Civil Twilight
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon and ends at sunrise.
Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon.
Under civil twilight:-
Horizon is clearly visible &
Terrestrial objects are easily perceptible without artificial light.
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TWILIGHT
Nautical Twilight
Nautical twilight is time when the center of the sun is between 6° and 12° below the horizon.
Nautical twilight is the point where navigation via the horizon at sea is no longer possible.
Referred as "first light" before civil dawn and "nightfall" after civil dusk.
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TWILIGHT
Astronomical Twilight
Astronomical twilight is the time when the center of the sun is between 12° and 18° below the horizon.
End of astronomical twilight is the point where the sky is no longer illuminated by the sun and is dark enough for all astronomical observations.
Dimmest stars visible to naked eye will :-
Appear in evening once sun falls more than 18° below the horizon (i.e. when astronomical dusk occurs) &
Disappear when sun moves to within 18° of the horizon in morning (when astronomical dawn occurs).
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DAYLIGHT & TWILIGHT
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MOONRISE & MOONSET
Moonrise and moonset times are computed for exactly same circumstances as for sunrise & sunset.
Moonrise & moonset may occur at any time during a 24 hour period.
Possible for Moon to be seen during daylight & to have moonless nights.
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MOON
Facts about the Moon…
Moon changes its appearance & position in sky with approx 30-day cycle.
Unlike stars, Moon can also be seen during day.
Moon is always involved in eclipses &
Its position seems to be well correlated with Tides.
Understanding motion of Moon with respect to Sun and Earth will explain these phenomena.
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PHASES OF MOON
27.32 days
Moon orbits Earth in sidereal period of days.
Moon
Earth
Fixed direction in space
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PHASES OF MOON
Fixed direction in space
29.53 days
Earth
Moon
Earth orbits around Sun => Direction toward Sun changes!
Moon’s Synodic period (to reach the same position relative to sun) is days (~ 1 month).
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PHASES OF MOON
From Earth, we see different portions of Moon’s surface lit by the sun, causing the Phases of Moon.
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PHASES OF MOON
New Moon → First Quarter → Full Moon
EVENING SKY
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PHASES OF MOON
Full Moon → Third Quarter → New Moon
MORNING SKY
Waning
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PHASES OF MOON
New Moon rises at _____ and sets at ____ .
1st Quarter rises at _____ and sets at ____ .
Full Moon rises at _____ and sets at ____ .
3rd Quarter rises at _____ and sets at ____ .
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PHASES OF MOON
New Moon rises at sunrise and sets at sunset.
1st Quarter rises at noon and sets at midnight.
Full Moon rises at sunset and sets at sunrise.
3rd Quarter rises at midnight and sets at noon.
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TIDES
Tides are caused by the difference of the Moon’s gravitational attraction on the water on Earth.
Between the near side and the center of the Earth.
Between the center and the far side of the Earth
2 Tidal maxima
12-hour cycle
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SPRING AND NEAP TIDES
Sun is also producing tidal effects, about half as strong as Moon.
Near Full & New Moon, those two effects add up to cause Spring Tides.
Near first & third quarter, two effects work at a right angle w.r.t. each other, causing Neap Tides.
Spring tides
Neap tides
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ECLIPSES
Parties involved: Sun, Moon, and Earth
Motion of the Moon around Earth:-
29 ½ day revolution of Moon around Sun
Tilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic plane (Earth’s orbital plane around the Sun)
The precession of the lunar orbit w.r.t. Earth-Sun direction
Solar eclipse happens when Moon is between Earth & Sun.
Size & distance of Moon need to be just right for us to see total eclipse:-
Changing distance between Earth and Moon explains occurrences of the total and ring solar eclipses.
Changing distance between Earth & Sun, and Earth & Moon explains difference in duration of solar eclipses.
Lunar eclipse happens when Earth is between Moon & Sun.
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CAUSES OF ECLIPSE
Earth and Moon cast shadows.
When either passes through the other’s shadow, we have an Eclipse.
Because the Sun is an extended bright object, there are two different regions of the shadow:
Penumbra is partially illuminated
Umbra is completely dark
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SHADOWS AND ECLIPSES
Sun
Penumbra
Umbra
Earth
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25. PRECESSION OF EARTH’S SPIN AXIS
5. Spin axis now points to Polaris. 13,000 years from now, Vega will be our “pole star”
4. Earth responds to this pull by slowly “precessing” its spin axis around a circle in the sky once every 26,000 years
23.5o
2. Moon’s orbital plane is tilted by 5o from our equator
3. Moon’s gravitational pull on Earth attempts to pull bulge into lunar orbital plane
1. The rotation of the Earth distorts it into an “oblate” spheroid flattened at the poles
IFC - Astronomical Data

26. ECLIPSE OF THE MOON (LUNAR ECLIPSE)
Earth’s Orbit
Moon’s Orbit
Occurs at Full Phase when Moon is also at “line of nodes” of its orbit
with respect to the ecliptic
Lunar eclipse lasts for many hours & can be seen from majority of
Earth’s surface
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ECLIPSE OF THE SUN (SOLAR ECLIPSE)
Earth’s Orbit
Moon’s Orbit
Occurs at New Phase when Moon is also at the “line of nodes” of its orbit
with respect to the ecliptic
Solar eclipse lasts for only for a few minutes and can only be seen from very restricted locations on the Earth’s surface
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SOLAR ECLIPSES
Sun has approx. the same angular diameter of ~ 0.50 as the Moon.
When the Moon passes in front of the Sun, it can cover it completely, causing a total Solar Eclipse.
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29. SOLAR ECLIPSE FORECAST
Solar eclipses from 2004 to 2030
Knowing the orbit of the Earth and the Moon, we can now calculate the time and path of solar eclipses with great accuracy.
IFC - Astronomical Data