Uday is the rise of the sun, suryodaya, and that of the moon, chandrodaya, play an important role in the horoscope of a person. Uday means coming up to a level. The sun’s rise comes into view two minutes earlier than the actual timing of rising because sun rays undergo refraction. The sun even rises when it is actually half a degree below the horizon. Now, at this virtual rise of the sun, the other planets are yet to rise. So, with reference to other planets, the day has already begun.
The same phenomenon happens when the sun sets. Due to the refraction effect, the sun sets two minutes later than the actual timing. Thus the day is increased by four minutes in all.
The period from one sunrise to another at the same place is called a day. Hence there is a difference between a day at a place and the twenty-four-hour-day of a clock. This difference is called velantara. The velantara, the precession of the sun at the vernal equinox (chara period)and the degree of refraction at a place, can determine the sun’s rise at that place.
The moon rises in the east after the sun sets. On the new moon day, the sun and moon are in the same nakshatra (asterism). When the moon is at twelve degrees away from the sun and the first tithi, pratipada, of the next lunar month starts, the moon’s rise occurs and a cusp of the moon can be observed.