Solar Limb Darkening Effect

Limb darkening refers to the diminishing of intensity in the image of a star as the star moves from the center of the image to the edge or "limb" of the image. Limb darkening occurs as the result of two effects:

1.    The density of the star diminishes as the distance from the center increases.

2.    The temperature of the star diminishes as the distance from the center increases.

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. They usually appear as pairs, with each sunspot having the opposite magnetic pole to the other.

Although they are at temperatures of roughly 2700 - 4200°C, the contrast with the surrounding material at about 5,500°C leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot is isolated from the surrounding photosphere it would be brighter than the Moon. Sunspots expand and contract as they move across the surface of the Sun and can be as small as 16 kilometers (10 mi) and as large as 160,000 kilometers (100,000 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope.

Solar Flares

A solar flare is a sudden brightening observed over the Sun's surface or 25 the solar limb, that is interpreted as a large energy release up to 6 × 10 joules of energy (about a sixth of the total energy output of the Sun in every second). The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly "active" to less than one per week when the Sun is "quiet", following the 11-year cycle (the solar cycle). Large flares are less frequent than smaller ones.

Solar Radiation Spectrum

Solar radiation is partly absorbed, scattered and reflected by molecules, aerosols, water vapor and clouds as it passes through the atmosphere. The solar beam arriving directly at the earth’s surface is called direct solar radiation. Direct solar radiation is observed from sunrise to sunset.

Using a solar radiation scanning spectrometer solar radiation spectrum is continuously monitored and recorded from a range of 350 - 1100nm. With the help of motorized solar tracker, solar spectrum in various zenith angles can be recorded (looking vertically, the zenith angle is 0°, and is 90° at the horizon).

The software developed for the instrument has facilities for setting up and calibration in addition to scanning and manual readout. Scanning is possible for desired wavelength range as well. This feature can save time whenever full wavelength range scan is not necessary. The software saves and displays the data acquired on MS Excel sheet. This helps to plot graph with ease whenever required.

Atmospheric Extinction Study

Atmospheric extinction is the reduction in brightness of stellar objects as their photons pass through our atmosphere. Two different mechanisms contribute to extinction: absorption and scattering. Normally, most of the extinction in the Earth's atmosphere is due to scattering; absorption becomes important when the air is full of smoke. Extinction is much stronger in blue light than in red light. This effect is usually called “reddening”.

Extinction values for different wavelengths in various angles (looking vertically, the zenith angle is 0°, and is 90° at the horizon) can be measured using Solar scope with tracker and narrow bandpass interference filters. Four interference filters can be accommodated in the Solar scope.

Measurement of Sunshine Duration

Sun photometer unit with tracking mechanism is used for this application. Using photodiode the light intensity of solar radiation is measured continuously. Although the radiation received by the photodetector contains both direct solar radiation and diffuse sky radiation, the latter is removed by differentiating the output signal.

Perihelion & Aphelion and Perigee & Apogee Measurements

Aphelion - perihelion and perigee - apogee. Both mean different things. Perihelion and aphelion is about the minimum and maximum distance between Earth and Sun, and the perigee and apogee, refers to the minimum and maximum distance between the Moon and Earth.

Distance between Earth and Sun

The average distance is about 150 million kilometers (149.6). This distance of 149.6, is defined as an astronomical unit or 1 AU, and is equal to about 8.2 light minutes, because light from the Sun, takes about 8 minutes to reach the Earth. In fact Earth’s orbit is not perfectly circular, so the distance between Earth and Sun varies slightly over the year, from a minimum of 147,100,000 km to a maximum of 152,100,000 km. So semimajor axis of Earth's orbit is 149597887.5 km (149.6). When closest to the Sun (or perihelion), the planet is 147,098,074 km, or 0.98 AU. At the farthest point from the Sun (or aphelion) is 152,097,701 km or 1.02 AU.

	Moon		Sun
	At Perigee (nearest)	At apogee (farthest)	At Perihelion (nearest)	At aphelion (farthest)
Mean radius	1,737.10 kilometers (1,079.38 miles)		696,000 kilometers (432,000 miles)
Distance	363,104 km (225,622 mi)	405,696 km (252,088 mi)	147,098,070 km (91,402,500 mi)	152,097,700 km (94,509,100 mi)
Angular diameter	33' 30" (0.5583˚)	29' 26" (0.4905˚)	32' 42" (0.5450˚)	31' 36" (0.5267˚)
Apparent size to scale

Transit of planet

A transit of planet across the Sun takes place when the planet passes directly between the Sun and Earth, becoming visible against the solar disc (and hence obscuring a small portion of it). During a transit, planet can be seen from Earth as a small black disc moving across the face of the Sun. The duration of such transit is usually measured in hours. A transit is similar to a solar eclipse by the Moon.

Solar eclipse

As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks ("occults") the Sun. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth in an alignment. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.

A    Total eclipse in the umbra

B    Annular eclipse in the antumbra

C    Partial eclipse in the penumbra