Today day and night are equal.

Astronomical autumn officially began with the autumnal equinox early this Wednesday (September 23). 

According to the Royal Museum Greenwich, the equinox occurred at 1:06 am UK time on Wednesday (September 23). This is 6:06 am Bangladesh time. 

An equinox is a time when the sun is exactly over the Earth's equator. At this time, no part of the Earth is tilted towards or away from the sun, causing the length of day and night to be approximately equal around the world. This event is called the 'autumnal equinox'. 

On this special day, people gather at historic sites like Stonehenge in the UK to enjoy the sunrise.

However, the situation is a little different in Bangladesh. Autumn is not starting again in Bangladesh on September 23. According to the Bengali seasonal cycle, autumn is already underway in Bangladesh. 

The name 'autumnal equinox' is primarily related to the astronomical cycle of the Northern Hemisphere. In many countries in the Northern Hemisphere, including Europe, this day is considered the astronomical beginning of autumn. 

It is worth noting that the Latin word 'equinox' means equal night. According to the rules, both day and night should be 12 hours long on this day. However, due to the refraction of light in the atmosphere, light remains for some time even after sunset. As a result, the event of the length of day and night being completely equal occurs just a few days after the equinox, which is called 'equilax' or 'equal light'.