What is meant by Uttarayanam ?

When the  Sun God enters the sign of

 Capricorn ( in the sign of Makara ) and

till He enters the zodiac of Cancer (Karkataka rasi) ,

 that six months period is called the Uttarayanam.

 In this period the Sun travels towards

 the north from the Equator (Bhumadhya rekha).

On this day it is most popularly known as

the Makara Sankramanam which is

 the harvest festival of Sankranti or Pongal.

In general the Sun stays thirty days in

 one sign of the zodiac like Aries, Tarus, Gemini etc.,

What is meant by Dakshinayanam ?

When the Sun God enters the house of

 (sign of) the zodiac of  the Caner (Karkataka Rasi)

and till  he enters the sign of Capricorn,

 that six months period is called the Dakshinayanam.

In this period the Sun travels towards the South from the Equator.