Visualize through this image. All calculations are made from the earth and we perceive Sun and constellations to be moving, but in reality Sun and distant 27 constellations of eastern horizon belt are static.
Earth takes around 360 days(360 degrees) to revolve around Sun. So, Sun remains in a sign made by 27 constellations/12 houses for one month and moves around 1degree across constellation every day as seen from earth. Makar Sakranti or Uttarayan is when Sun enters constellation Makara(Capricorn) as seen from earth. The current Roman Gregorian calendar of 365 days with random month span(28,30 or 31 days) and with a leap year every 4 years is total illogical calendar passed on to Christian nations by popes. Thats why birthdays of every being and their fates(Vedic horoscope) are controlled by spiritual astrology timelines not scientific material astronomy of modern atheist scientists who call it pseudo-science and deny the existence of spirit. Therefore, Hindu festivals like Diwali and birthdays of avatars like Ramnavmi, Jamashtmi which are according Vedic calendar fall on different days of Roman calendar every year. Actual new year for this earth is not January 1, which is some ancient local festival of Roman Saturnalia, rebranded as new year by Christians, but around mid-April Mesha Sankranti when Sun enters Mesha, the first house of zodiac belt and celebrated as Vaishaki or crop harvesting time as earth enters Vishaka constellation at the same time.
In Vedic astrology the zodiac is centered on the Sun. So Sunrise time
at the place of birth is important. This is what determines your Lagna
or the ascendant, which is the central keystone of your horoscope.
As Earth takes 24 hours to spin around its axis, all the 12 zodiac
signs will rise on the eastern horizon. So in a day there will be 12
Lagnas. Hence 24 hrs divided by12 = 2 hours is the time span of each
Each sign has 30 degrees. Hence 2 hours = 120 minits divided by 30
degrees = 4. So every 4 minits the Lagna increases by 1 degree.
When the place of birth changes, the latitude and longitude also
change, sometimes the time zone also changes and because of this the
Sunrise time also changes accordingly. This is the reason why Lagna
changes from place to place.
So, Lagna is based upon the Sunrise time of location and birth time of the person and date(controlling the sign in which Sun is present). Only those born at Sunrise has Sun in their lagna with Sun Lagna house increasing/rotating sign every 2hrs from Sunrise. While rashi is controlled by Moon sign or Moon projected over constellations.
In the shared example of the horoscope, the person is born when Sun is in 2nd house i.e around May and Sun is in 7th house, so time of day is around evening and Sun is ahead of Moon by 4 houses, so in mid of Krishnapaksha.