The Sun and Moon as a pair
The Sun and Moon are the two luminaries and form the basic pair of Jyotish. Their relative position gives the tithi, and their combined longitude gives the yoga.
What it is
The Sun represents the self and authority, and the Moon the mind and feelings. In the calendar, the Moon's distance from the Sun defines the lunar day and the phase.
How it is worked out or read
The Sun and Moon are the two grahas that never retrograde, and both are used in the panchang at every sunrise.
Where it appears on this site
See tithi and panchang yoga.
The two lights
The Sun and Moon are called the two luminaries, and in Jyotish they hold a place above the other grahas. The Sun, Surya, is read as the soul, vitality, authority and the father. The Moon, Chandra, is read as the mind, feelings, nourishment and the mother. Together they describe how a person's inner core and inner responses work with each other. The Sun rules Simha, is exalted in Mesha and debilitated in Tula. The Moon rules Karka, is exalted in Vrishabha and debilitated in Vrishchika.
In the natural friendship table, the Sun counts the Moon as a friend, and the Moon counts the Sun as a friend. The Moon is unusual in having no natural enemies at all.
The relationship that makes the calendar
The angle between the Moon and the Sun is the basis of the whole lunar calendar. Each tithi is a stretch of twelve degrees in that angle. When the Moon and Sun are together, the angle is zero and it is Amavasya, the new moon. When they are opposite, the angle is 180 degrees and it is Purnima, the full moon. The yoga of the panchang is built from the sum of their longitudes, and the karana is half a tithi. So three of the five limbs of the panchang come directly from the Sun-Moon relationship.
This is a reminder that much of Jyotish starts from simple, observable astronomy. Anyone who watches the Moon's phase is watching the angle that defines the tithi.
The relationship in a birth chart
In a chart, the reader can look at the house of the Moon counted from the Sun. A Moon far from the Sun, in the waxing half and bright, is traditionally treated as strong, while a Moon very close to the Sun is weaker in light. Some texts consider the Moon beneficial when it is waxing and more mixed when it is waning. The birth tithi, which a reader can find directly from the chart, therefore gives a quick sense of this relationship.
The houses ruled by the two luminaries also matter. For Karka Lagna the Moon rules the first house and the Sun the second; for Simha Lagna the Sun rules the first and the Moon the twelfth. Their placement relative to each other then links those houses.
A balanced reading
Popular readings sometimes say that a person's Sun sign is their outer self and their Moon sign their inner self. Jyotish gives the Moon sign much more weight than Western sun-sign astrology does, because the Moon governs the nakshatra, the dasha and many daily readings. A balanced approach notes both luminaries, their signs and dignities, the tithi between them and their houses, and then reads them together rather than treating either as the whole story.