The Sun and Moon are referred to as luminaries since they cast light.  Thus, when they are aspecting a planet they will shed light on them and highlight the planets' influence.  While the Moon is associated with emotion, most astrologers believe that the Sun has no specific influence of its own.  Rather, it simply throws light upon whichever Sign it happens to be in and triggers its effects as described in the Sun Sign section where you can find an overview of all the Signs and links to individual descriptions for each.  The Sun rules Leo but is influential for all as a personalized planet.  In Vedic astrology, which originated in India, the Sun is considered malefic, or a stressful influence.

As the Sun travels through the Zodiac it will highlight/trigger events related to that particular House and you can expect extra activity in that area.  If look back on major events in your life you'll find that more often than not this is the case.  For example, if you took a long trip it's very likely the Sun was in your 9th House, or possibly a New or Full Moon occurred there as lunations likewise highlight that area of your life.  If you got a new job or had surgery, most likely it was in your 6th House; redecorating your home, in your 4th House; started a new relationship, in your 7th House, etc. 

When the Sun is close to another planet on your natal chart it can have a variety of effects.  When the Sun is within 17 minutes of arc (00:17) of a planet it is in an aspect known as cazimi, or in the heart of the Sun.  In this case the Sun and planet join forces with the Sun projecting the energy of the planet in a powerful way.  For example, some with Venus cazimi would project a very caring persona and draw people to them.  Oprah Winfrey has this aspect.  If a planet is past cazimi but within 8 1/2 degrees of the Sun it is considered combust.  In this case the Sun is overpowering the planet, essentially burning up its affect, inhibiting its ability function independently of the Sun.  If a planet is past being combust but with 16 degrees of the Sun it is considered "Under the beams" which is slightly less inhibited than being combust but still unable to function entirely on its own, as if the Sun's glare is preventing it from manifesting itself fully.

When the Sun returns to its exact position where it was when you were born, this is called the Solar Return.  A Chart cast at this moment indicates areas of focus for the following year, with a few months overlap at the beginning and end as one year fades into the next.  See our Reports section, which contains a link to a sample report, if you're interested in learning what this year holds for you.

Solar eclipses often have a significant affect as well and further emphasize the Sun's influence in that particular House, especially when the eclipse occurs within a few degrees of a natal planet.

See an awesome picture of the surface of the Sun here.

A Solar Return Report tells you what to expect in the year following your birthday.

Can you name the planets in order from the Sun?