Annular solar eclipse,
June 21, 2020

Eclipses require that the Sun, Moon and Earth be in a straight line (so that the shadow of one can fall on the other). Hence they can only happen on New Moon (amavasya) or Full Moon (poornima). Since June 21, 2020, is an amavasya, the Moon can come between the Sun and the Earth on that day. Its shadow falls on the Earth, giving us a solar eclipse.
The plane of the Moon's path around the Earth is tilted at an angle of 5o to that of the Earth's path around the Sun, as shown in the Figure. So on most New and Full Moons, the Sun, Earth and Moon do not fall in a straight line. Because the Moon's path is at an angle, only part of it may fall in the Earth's shadow. This gives rise to a {partial} lunar eclipse. For a {solar} eclipse to be partial, there is an additional reason. The Moon's shadow on Earth is very small. So a central solar eclipse will be partial when seen from most places, and central only on a narrow path.
Eclipses are classified as:
{partial} when the Moon only partially obstructs the Sun, and 
{central} when the Moon maximally obstructs the Sun.
Central eclipses are {total} when the Moon completely obstructs the Sun, and {annular} when a thin ring of the Sun's disc is not covered by the Moon.
Because the Earth--Moon distance varies, the Moon need not cover the Sun fully during a central solar eclipse. When the Moon is farther enough from the Earth we get an {annular} eclipse. When the Moon is close enough to the Earth we get a {total} eclipse.
On 21 June 2020, we have an annular eclipse of the Sun. The path of annularity is very narrow. It passes through Suratgarh in Rajasthan, Sirsa and Kurukshetra in Haryana, Yamuna Nagar in Uttar Pradesh, Dehradun and Joshimath in Uttarakhand at around 12:10. 99.7% of the diameter of the Sun will be covered for
38 seconds at Joshimath.
Most of India will see a partial eclipse starting from around 10 or 11 am for 3 hours or more, ending at 1:30 or 2:30 pm. Bikaner and Delhi will see 95% of the solar surface covered by the Moon, Kolkata and Mumbai about 70%, Chennai just about half and Kanyakumari about one-third.

Pin-hole images
Monitoring the progress of the eclipse will require seeing the partially eclipsed sun. An interesting phenomenon to observe during partial phase is the ``Pin-hole'' camera effect created by the leaves of trees as sunlight filters through them (more precisely, through gaps in them). Before the eclipse begins, the ground is covered by overlapping discs which are actually the images of the sun. As the eclipse progresses, these discs take the shape of the eclipsed sun. When the eclipse is quite advanced, one sees a myriad collection of crescents! Spread a white sheet on the ground to enhance the effect.
Looking at the Sun, either directly or through your camera, can burn your eyes and cause blindness. Never look at the Sun without adequate protection. Protecting your eyes adequately means reducing exposure to ultra-violet and infra-red radiation, which can damage your eyes instantaneously without your immediately being aware of it. Also, adequate protection will increase eye comfort by reducing the intensity of sun's visible rays.
The intensity of sunlight for direct safe viewing should be reduced by at least 100,000 times and ultra-violet and infra-red part of the solar radiation should be effectively cut off. Therefore always use a filter that will absorb equally and sufficiently the ultra-violet, infra-red and visible energy of the Sun.
There have been wrong recommendations suggesting the use of materials that absorb the visible energy but do not absorb the dangerous, invisible infra-red rays. These days filters are sold by many commercial platforms. But ensure their safety by reading reviews about the products. Don't always go for cheaper alternatives, safety comes before savings!
One can also observe by projecting the image of the Sun. This arrangement should work quite well, it is sometimes mounted on a ball: cover a plane mirror with a piece of paper having a circular hole of diameter 1 to 2 cms. The sunlight reflected is thrown on to a shaded wall indoors. Reduction in the diameter of the hole in the paper will increase the sharpness of the image at the loss of its brightness.
Alternately, one can just build a {pinhole camera} with two plane cardboard sheets. In one, make a hole that is not too small (smaller the hole, less bright will be the image), nor too big (bigger the hole less sharp will be the image). A separation of 1 metre can produce an image of about half a cm.
Based on the manual by N.D. Hari Dass and Kamal Lodaya