It will take between 1 and 6 million years to travel 100 light years. If you wanted to travel 100 trillion light-years away, you could make the trip in 62 years. By the time you arrived, the Universe would be very different. Most stars would have died long ago, the Universe would run out of usable hydrogen.
You would have left a living and thriving universe billions of years ago. And you could never come back. Traveling 100 light-years at 0.25°C would take the astronaut 400 years, but it would take 413 years for the observer. At 0.9°C, 111 years to reach 100 light-years, the observer would wait 254 years.
Proxima Centauri is 4.2 light-years from Earth, a distance that would take about 6,300 years to travel with current technology. Such a journey would take many generations. In fact, most of the humans involved would never see Earth or its exoplanetary counterpart. These humans would need to reproduce with each other throughout the trip in a way that ensures the arrival of a healthy crew to Proxima Centauri.
Since I was moving away from Earth, the Earth in my frame of reference is behind me, so the star must be closer than 60 light years and it will take me less than 75 years to reach it. Since the distance between cosmic bodies was mainly expressed in millions and billions of kilometers, a more convenient unit of measurement was needed to easily express such large distances, which led to the light-year being used as a unit for measuring astronomical distances. If you wanted to travel with a constant acceleration of 1G and then a deceleration to the very edge of the observable Universe. And from your close perspective at the speed of light, it also seems that I'm moving incredibly slowly, everything is relative.
Space is crushed in the direction in which you travel, so, for you, you didn't travel as far as the stationary observers saw you traveling. Forty years later, he used a mile-long depressurized corrugated steel tube to simulate a nearby vacuum and obtain a better measurement, which was only slightly lower than the accepted value of the speed of light today. If you accelerate at that rate for years, you can travel billions of light years over a human lifetime. And it took almost a decade for New Horizons to get from Earth to Pluto, which is just around the corner, 4.6 light hours away.
In order for scientists to correctly calculate the light-year, it was important that they had the value of the speed of light. In 1676, Danish astronomer Ole Rømer accidentally calculated a new estimate of the speed of light while trying to create a reliable astronomical clock for sailors on the high seas.
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