Contrary to the popular belief, Makara Sankranti is not only a Hindu festival but actually a universal phenomenon because it is purely based on the science of astronomy & crop cycle which is not limited to any particular religion. Basically, it is a celebration of the “revival” of sunlight (solar energy & positivity) into our lives and it coincides with the harvest season as well.
Much
before contemporary scientists came up with their observations of solar
system a few hundred years ago, scholars of ancient India (5000 years
ago) already knew that sun is the center of our celestial system and is
the source of energy for all planets including our planet earth, which
takes 365 days to revolve around the Sun. This complete cycle of 360
degree which is also called the solar calendar is divided into 12 phases
or domains or zodiacs of 30 degree each (360/12=30) and each zodiac is
associated with certain characteristic depending on the positions of
earth & sun in relative to each other, which in turn also determines
the season & radiation patterns.
Since the radiations
are proportional to the amount of sunlight, an easier way to analyze the
radiation pattern would be by tracing the amount of sunlight received
each day, which again is proportional to the length of the day. If we
plot this on a chart with sunlight on Y axis & dates on X axis, we
will notice that the amount of sunlight varies in accordance with a sine
wave, with the trough (lowest point) in the phase corresponding to late
December and early January range.
Source:
http://astro.unl.edu/classaction/animations/coordsmotion/daylighthoursexplorer.html
In other words, it means the zodiac
phases of the Sun from July to December witness decreasing sunlight, and
after that, the subsequent zodiac phases witness increase in sunlight.
For an “observer”, it looks like the Sun is on a downward journey
between July to December and this downward journey suddenly changes to
upward journey or northward movement in late december & early
january. Since uttara means northward & aayana means movement in
Sanskrit, this phenomenon of phase reversal from southward movement to
northward movement of the sun is called “Uttara Ayana” or “Uttarayan” in
short.
Image courtesy:
http://www.tutornext.com/physics/appearance-of-the-sky.html
If the chart is divided into zodiacs,
then we can notice that it is Makara which is the first among the series
of phases witnessing an upward journey, and this trend continues with
subsequent phases till peak summer. Hence the Makara phase holds such
great significance in astronomy because it represents revival of
sunlight & energy.
But if Makara is the name of a zodiac,
then what does Sankranti mean? In Sanskrit, Sankranti means the entry of
Sun into a zodiac phase. Whenever the Sun moves out of one zodiac phase
& enter another zodiac phase, such transmigration of the Sun is
called a Sankranti. Since we have 12 zodiacs in an year, there will be
12 Sankrantis each year. But as we saw how Makara is significant due to
the revival of sunlight, the “Sankranti of Makara” (or transmigration of
Sun into Makara) is celebrated as “Makara Sankranti”, which usually
falls on 14th of January (or 15th on certain years).
Apart from the astronomical & solar
significance, this co-incides with the harvest season as well, and is
hence celebrated with lot of pomp and show by almost all communities
througout India, albiet in different names & slightly varied
traditions but the core logic is the same. It is celebrated as Lohri in
Punjab, Uttarayan in Gujarat, Makara Sankranti or Makara Sankramana in
Karnataka & Maharashtra, Pongal in Tamil Nadu, Khichdi in Bihar,
Bihu in Assam. Almost all regions in India celebrate this festival,
cutting across all castes & creed because it is nothing but a
tradition to observe & celebrate the revival of solar energy in our
lives. i.e Irrespective of our religion, we all benefit from solar
energy. Also, this coincides with the celebration of crop harvesting
resulting in continuity of food supply which irrespective of our
religion, is one of the most basic necessities of our lives.
Wishing everybody in this vast world
of Vasudhaiva Kutumbakam a very Happy Makara Sankranti, Lohiri, Bihu,
Pongal, Kichdi, Uttarayan. 
