Mercury is 36 millions miles away from Sun ; Venus is 67 million miles away from the Sun.
Therefore, there will be occasions when Moon, Mercury and Venus can come between the earth and the Sun.
Suppose you keep one of your fingers near either of your eyes. Close the other eye. One elephant standing 100 meters away is hidden by this small finger. This depends on the size of the finger, its nearness to the eye and the farther distance of the elephant. Now, your eye, the finger and the elephant are in the same line and plane. Suppose, the elephant is lifted by a lift, in the same place. Even though the elephant was hidden by the finger previously, as it is lifted up above the area covered by the finger, then the elephant becomes visible, slowly and steadily.
Thus Moon is a decently big body. It is very near to the earth. The Sun is very far off. It is very big like the elephant compared with finger. So, when the Moon comes in between the observer and the Sun, the Sun is not visible. Slowly the shadow of the Moon, covers the Sun. This is called the Solar eclipse. During the duration of the eclipse the declination of the Moon and Sun will be the same. But if the declination is different, but the celestial longitude of both is the same then it is only New Moon. Similarly, when earth comes between Moon and Sun, earth prevents Moon from getting the light of the Sun, and lunar eclipse is formed.
Just like, during Solar eclipse, Sun loses its brilliance and is hidden by Moon and during lunar eclipse Moon loses its pleasant light and is hidden by the earth, so also the rays of other planets are interrupted. Because, we, on the earth, do not receive very bright light, illuminating the objects on the earth, from other planets, we cannot see, notice and realise the eclipse of the other planets.
The planets Mercury and Venus are called Inferior planets as they are nearer to Sun than the earth and they can be between the Sun and the Earth, just like Moon during New Moon days and Solar eclipse days. Moon is hidden from Sun by the earth during lunar eclipse. But earth can never come in between Mercury and Sun or in between Venus and Sun. So Earth can never eclipse Mercury or Venus.
What eclipsed planet can do in its period or sub-period: (as per KP)
Sub lord of 1st cusp : Let us consider the results of the planets situated in the constellation of those which are eclipsed and what they signify. If a planet is deposited in the constellation of an eclipsed one and if it is the sub-lord of the lagna cusp, the sub lord shows that the native will be imprisoned; he will lead the life incognito; will suffer from fear, disease and anxiety, will participate in funeral rites; incur loss and may lose position too, will be unfortunate, etc. .
Sub lord of 2nd cusp : Behaves stupidly in a public assembly, will be untrustworthy, will receive annoying letters, write bad ones or anonymous ones suffer from eye troubles, will be foul-tongued, spend heavily, incur royal displeasure.
Sub lord of 3rd cusp : Demise of brother, evil advice, trouble through secret mechanisation of internal enemies, humiliation, discomfitures and loss of pride thereby.
Sub lord of 4th cusp : Distress to mother, sickness to dear friends, danger to building, trouble due to the loss of cattle, danger from water and conveyance.
Sub lord of 5th cusp : Loss of a child, aberration of the mind, deception, wearisome, wandering, trouble in the stomach, displeasure of the Government and bodily weakness.
Sub lord of 6th cusp : Will have troubles through thieves, will suffer reverse, defeat, will commit wicked acts, will do servile duties, will be despised, scandalised and injured.
Sub lord of 7th cusp : Distress to son-in-law, separation from beloved, untoward results due to the troubles from the Opposite sex, intrigues with wicked women, diseases in the private parts, ever wandering.
Sub lord of 8th cusp : Excessive sorrow: want of sense, carnal appetite, jealousy, unconsciousness, poverty, fruitless rambling, sickness, humiliation and death.
Sub lord or 9th cusp : Will incur the wrath of the deity or Jupiter or preceptor or father suffer something untoward, trouble to wife and children, will do wicked acts, elder’s demise, will suffer from poverty.
Sub lord of 10th cusp : Fruitless attempts, loss of honour, will become wicked, leaves his permanent place, inauspicious happenings, will lead bad life, suffer troubles.