One must remember that the planets don't stop moving after the day one is born. They keep on circuiting the heavens and influencing one's affairs by their motion continuously throughout life. When a planet travels in the sky through the different signs of the zodiac, it will be influencing those signs in your horoscope. These planets temporarily affect the affairs corresponding to the house they are transiting. The study of these transiting planets is used in conjunction with the main Vimsottari predictive system.
Transits of the Moon (the fastest moving planet) will give its changing influence on a daily basis, as it takes approximately two to three days for the Moon to transit a sign. This is why the Moon is the most important planet for predicting our daily affairs. The Sun takes one month to transit a sign. Mars, Venus and Mercury can take anywhere from two to six months.
Apart from using the Moon to predict daily events, the most important transits are those of Jupiter, Saturn and the nodes Rahu and Ketu. These planets take a long time to travel through a sign and give long lasting results. Transits will often trigger important events that are seen in the Dasa Bhukti, when the transiting planet touches the same house or planet giving results in the Dasa Bhukti. Transits are read from the Moon and Ascendant. They will show changes in our psyche (Moon) and in our outer lives (Ascendant).
Jupiter takes a year to transit a sign and its transit over a house/sign can give important growth, opportunities or success in that particular area of life. Jupiter is a natural benefic and will bring expansive effects to the house it is transiting. Jupiter also signifies husband in a female's chart, or children and when transiting the 5th, 7th house or their lords can trigger marriage or the birth of children. Every 12 years or so Jupiter transits over natal (position of planet at birth) Jupiter. This is called Jupiter Return and often triggers some important event. It brings luck, happiness and fortune.
Rahu transits a new sign every year and a half. Rahu will also bring unexpected opportunities, sudden results, or untapped potential to the fore. Every 19 years or so Rahu transits over natal (position of planet at birth) Rahu. This is called Rahu Return and it helps trigger the future growth or potential of our Rahu in the birth chart and usually marks an important time in our lives.
Saturn is the slowest moving planet and takes around twenty nine years to transit all the signs of the zodiac. It will stay in a sign for two and a half years. Saturn gives the most dramatic and life-changing effects during its transits, particularly when it passes through certain sensitive points in the chart.
Saturn will often concretize the affairs of life represented by the house and planets it is transiting. It will often bring important life changes such as marriage, embarking on a radically new direction in life, or reaping the results of one's efforts. If a person has not utilized their Saturn energy well in the horoscope, in terms of working hard towards their goals or living in a way that is true to one self (Saturn being a planet of reality, genuineness), then Saturn's transit is likely to cause significant upheavals and inner changes. Saturn being a natural malefic will always give some difficulties and struggles to the house or planet it is transiting depending on the strength of that house or planet in the chart. Saturn finally is the planet of karma and time. You cannot escape Saturn. Whatever you have been doing in those affairs represented by the house Saturn is transiting will surely give its results positive or negative during Saturn's transit.
The most important transits of Saturn are:
Saturn Return - Every 29 years or so transiting Saturn passes over the natal Saturn in your chart. This is known as Saturn Return. This event tends to make a person seriously question and evaluate everything they have been doing in their lives up till then. It can signify a strong need to move in a completely new direction, or a person begins to reap the good or bad results of whatever they have been working towards up till then. If they have been avoiding the path they know they should be following in life, when their Saturn Return arrives, they will definitely have to rethink and adopt a different way of life, either voluntarily or it will be forced upon them. Also the affairs of life that natal Saturn governs in the chart will play a major role in their lives.
Sade Sati - Saturn transiting the Moon is dreaded in Hindu Astrology and is called Sade Sati. Although it has a poor reputation it isn't necessarily bad. It's effects however are profound and enduring.
Sade Sati is a seven and a half year period beginning when Saturn transits the sign prior to the Moon. Saturn will then transit the sign the Moon occupies and the sign after the Moon. It generally occurs twice or thrice in a person's life. It causes enormous inner changes on a person's psyche (Moon) and can be a great period of progress in a person's life, (if the Moon is strong in a chart and has a favourable relationship with Saturn) or it will bring inner turmoil, greater difficulties and necessary but sometimes painful lessons. The seven and a half year period of Sade Sati is definitely an uphill climb in the chart. Saturn makes this period a struggle, presents obstacles and deeply concretizes certain aspects of a person's life or their inner being.
The first Sade Sati period is said to be a little difficult giving some personal crisis or mental confusion. It will usually give difficulties if it occurs in childhood. It will give some crisis in regards to the parents (particularly mother) and sickness in infancy.
The second Sade Sati period is a time when a person can often experience the death of a parent, or it can involve one deeply in their economic development and worldly occupation.
The third Sade Sati period often can give strong spiritual inclinations (ascetism), withdrawal from worldly life, physical illness and death. These periods can be very rewarding as well as troublesome and are likely to bring major inner and outer changes, which have life long effects.
This is a simple way of analyzing transits.
There are more complicated methods of judging transits based on Ashtakavarga.
This is a system where points are assigned to each of the houses. When a planet transits a house with high points, it is said to give good results, even if the planet is malefic in nature, it will not ruffle the affairs of a house too much. When a house has low points, transits by malefics can cause some harm to the house. If the transitting planet is a benefic in the case of Jupiter, it may not give as much benefic results due to its transit of a house low in points. The system also assigns points to each planet relative to a particular sign. When a planet is transitting a sign where the planet attains high points it can give powerful results. This system does not confer its results in an obvious and simple way, and one has to balance it with the basic methods outlined above.
One can find out the current transits of all the planets by getting hold of a sidereal ephemeris such as "Lahiri's" ephemeris or what is called a "Panchang". Most Vedic Astrology software programs include a daily ephemeris. One can also construct a birth chart for the moment to check the current planetary positions.