For example, dividing the zodiacal circle in half gives the opposition aspect (180 degrees). This is when two planets are on opposite sides of the Earth creating a 180 degree separation. A Full Moon is represented by this aspect, when the Sun and Moon are facing each other and the earth is the centerpoint. When two signs are 180 degrees apart they are said to be in polarity.

The reverse of the opposition is the conjunction. This occurs exactly when the two planets are in the same degree of longitude in the zodiac. This is known as the New Moon In Sun Sign astrology and is when the Sun and Moon are in the same sign.

The following tables provide the other major aspects.

The Conjunction -- 0 degree
when two planets are in the same degree of longitude (or within orb), gives great strength to the energies of the interacting signs and planets. This aspect can be either harmonious or discordant, depending upon the nature of the planets involved. This aspect is also known as the 1 Vibration.

Signs that are conjunction to each other are:

The Semi-Sextile -- 30 degrees
This aspect is usually harmonious which can be reached after compromise.It occurs one sign before or one sign after your own. There is a karmic link to this aspect. The semi-sextile is known as a minor aspect and the 2 Vibration.

Signs that are semi-sextile to each other are:

The Sextile -- 60 degrees
is a favorable aspect, the planets usually being in congenial signs of compatible elements. This aspect can allow the influences of the planets to work in harmony. However, the sextile only brings opportunity. These opportunities must be acted upon to be of use. This aspect is also known as the 3 Vibration.

Signs that are sextile to each other are:

The Square -- 90 degrees
is traditionally regarded as unfavorable. It represents the struggle of two forces at cross purposes. The square brings stress and denotes obstacles to inspire growth through concentrated effort. This aspect is also known as the 4 Vibration.

Signs that are square to each other are:

The Trine -- 120 degrees
is the most harmonious aspect. In most cases, it joins planets in conjenial signs of the same element. These energies combine with ease. The drawback is the lack of challenge -- getting benefits without effort. This aspect is also known as the 5 Vibration.

Signs that are trine to each other are:

The Quincunx -- 150 degrees
Is also known as the inconjunct and has karmic lessons to teach us. There is a certain uneasiness about this connection and a feeling of discomfort. The inconjunct is known as a minor aspect and is also known as the 6 Vibration.

Signs that are quincunx to each other are:

The Opposition -- 180 degrees
when planets are exactly opposite each other in the zodiac, causes stress and awareness. This polarity, though tense, does not have to be discordant, as these opposites are complimentary. This aspect is also known as the 7 Vibration.

Signs that are opposition to each other are:

Orbs

It is a matter of personal opinion and experience in deciding whether or not two planets can be considered within the orb of a specific aspect.
The following is a quick guide for orbs and aspects.