Grahan means eclipse. There are two Grahan - Soorya Grahan (Solar eclipse) and Chandra Grahan (Lunar eclipse). Soorya Grahan always falls on Amaavasyaa (New Moon day) and Chandra Grahan always falls on Poornimaa (Full Moon day). Both periods are considered auspicious for Snaan (bath) and charity (Daan).

On these two days, Soorya and Chandra are devoured by Raahu and Ketu respectively. One of the names of Raahu is Bhoomi-Chhaayaa. Although in Indian astrology Raahu and Ketu are considered as planets but in reality they are not planets like other planets. They are not visible. They are called Chhaayaa Graha or shadow planets, that is why their name as Bhoomi Chhaayaa is not an inappropriate one.

If solar eclipse is on Sunday, or lunar eclipse is on Monday, it is called Choodaamani Yog. Bathing and donating during that time give eternal fruits.

Timings and Effects of Eclipses
Timings of Eclipses are normally fixed. Normal between one eclipse and the other, whether it is Solar or Lunar, is 6 months. Because, every 6 months Brahm etc 7 Devtaa are the Parvesh (Lords of Eclipse). Their names are - Brahmaa, Chandramaa, Indra, Kuber, Varun, Agni and Yam.

--Braahm Parv - If the eclipse is in Braahm Parv, there is increase in cattle, grains and Dwij.
--Chandra Parv - If the eclipse is in Chandra Parv, there is increase in cattle, grains and Dwij, but people suffer with Kaph
--Indra Parv - If the eclipse is in Indra Parv, there is conflict in kings, sorrows in the world, and destruction of crops.
--Varun Parv - If the eclipse is in Varun Parv, kings suffers while their people enjoy.
--Agni Parv - If the eclipse is in Agni Parv, there is enough rains, abundance of grains, and all are happy and comfortable.
--Yam Parv - If the eclipse is in Yam Parv, there is shortage of rains, loss of crops, and maybe there is famine too.

As the eclipse timings are set, if the eclipse occurs before the set time, there is loss of crops and kings are in danger; and if it occurs after the set time then there is loss of flowers, fear in the world, and destruction of crops.

If there are two eclipses in the same month, Solar and Lunar, then there is destruction of wealth and shortage of rains.

If there are total Solar or Lunar eclipses, there is famine, diseases and danger of fire in the world.

If the eclipse is in Uttaraayan (Jan 14 to Jul 14), there is loss to Braahman and Kshatriya; and if it is during Dakshinaayan, it harms other Varn people.

If one can see Raahu clearly, in various directions (North, East, South and West) of the shadow of the Sun and the Moon, it harms Braahman, Kshatriya, Vaishya and Shoodra respectively.
[Naarad Puraan, p 323-324]
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Why Mantra Saadhanaa is beneficial during Soorya and Chandra Grahan?
Manas is represented by Chandra and his Aatmaa is represented by Soorya, and whenever Manas and Aatmaa of a person align perfectly at some target or point the result is Samaadhi. During an eclipse, cosmic representatives of Manas and Aatmaa come in perfect alignment from the point of Earth where the eclipse is visible, therefore it is analogical to Samaadhi, and that is why eclipse is so important spiritually, and for Saaadhana. Mantra Siddhi become easy during this period.

Another reason is - Kundalinee Shakti travels through 3 paths : Idaa, Pingalaa, and Sushumnaa. This means : Sun, Moon and Fire [middle Naadee-meridian]. Thus when the Sun and Moon are together the fire Naadee Sushumnaa middle Naade is working very well and awakens the crown Charka, the 1008 petal lotus in head, more easily as naturally all 3 Naadee meridians are working well.

Mantra Saadhanaa During Solar Eclipse
There are two types of Saadhanaa which are recommended during Solar eclipse - Daan and Mantra Saadhanaa. Find out in which Nakshatra (star) the eclipse is falling. If it is one's birth star, one should take special care of themselves as per Indian Astrological guidance. They should observe these Saadhanaa. Daan (donation) may be of rice, wheat grains etc along with some vegetables and a few coins of their choice either before or after the eclipse.

The second Saadhanaa is more important Saadhanaa than Daan at this time. As per the ancient belief Saadhanaa on such a day will help the Saadhak in overcoming a number of problems including health related ailments, financial problems, problems concerning career and many such. What is needed for this, is a Kaam Roop Mani. This Mani is available in different shades. It may not be available in shops in the markets but someone who knows it and who can identify it can surely make a Praan Pratishthaa in it and activate it. In any case, even if this is not available, the Saadhanaa can be done to the possible extent.

Avoidable and Recommendable Works During Soorya Grahan and Chandra Grahan:
--It is advisable to avoid performing any important work or deal.
--After Grahan, many Hindu perform ritual bath. Take meal only after the ritual bath. Taking a holy dip in rivers or lakes, if possible, is highly auspicious.
--Many Hindoo observe fasting on the day of Soorya or Chandra Grahan.
--Avoid outside food during Grahan and as well as the whole day.
--Perform ritual bath, Poojaa, Jap, Havan and all other rituals only after Grahan Moksh Kaal - the time when Grahan is released.

In Soorya Grahan
--The very first caution - one should not go out while Soorya Grahan is taking place as far as possible.
--One must not look at the Sun with naked eye during the Grahan time in any case. If at all one needs to see it, he should use a piece of glass covered with the smoke of a Deepak (lamp) or dark sunglasses.
--Pregnant women must not come out of the room, sit inside, until the whole Grahan is finished. This is to prevent birth defects for their babies. There is belief in Hindoo society that a couple can do anything to ensure that their baby is not born on the Grahan day.
--Throw any cooked food and all drinking water if it remains, wash and clean utensils kept outside where they are affected by sunlight, before using them again.
--Donate as much as you can during this period.
--One should not lie or sleep also during this time. It is always good to sit on the floor and do Keertan or Jaap. One may chant Soorya Gaaytree Mantra, 1 Maalaa, 108 times at least, or more is good. Soorya Gaayatree Mantra is "Aum Bhaaskaraaya Vidmahe, Mahaa-tejaaya Dheemahi, Tanno Soorya Prachodayaat".