How we know
Start with a test you can run on yourself. Touch your nose and your toe at exactly the same time. You feel both at once. You shouldn't. The touch on your toe has to travel about a metre further up your nerves than the touch on your nose, and nerve signals are not instant. Conduction velocities catalogued in StatPearls run from roughly one metre per second in slow unmyelinated fibres up through much faster fibres, so the toe signal genuinely arrives at your brain later. Your brain quietly waits for the laggard and binds the two events, then reports them to you as simultaneous. David Eagleman's lab studies exactly this: how the brain stitches signals that arrive at different times into one "temporally unified picture of the world."
That waiting is not a glitch. It is how perception works for everything. Light hits your retina, the signal travels to your visual cortex tens of milliseconds later, and then more processing is needed before you consciously see anything. Inui and Kakigi 2006 traced the wave of activity spreading from the primary visual cortex outward after a simple flash. Amano and colleagues 2006, in the Journal of Neuroscience, used magnetoencephalography to time when a percept is actually established in the brain and confirmed it trails the physical event. Put together, the moment you call "now" is already a fraction of a second, on the order of a tenth, in the past.
So you are never watching reality live. You are watching a replay your brain finished assembling a beat ago.
What it means
The lag is tiny up close, but it never goes to zero, and it scales without limit the further away you look. The sun you see is not the sun as it is now. NASA puts the travel time of sunlight at about 8.3 minutes, so you are always looking at an eight-minute-old star. If the sun simply switched off, you would carry on seeing it shine for over eight minutes before the sky went dark.
Go further out and the delay becomes centuries. The stars in the night sky sit hundreds to thousands of light-years away, so their light left long before it reaches your eye. Some of those stars have already collapsed or exploded. You are still seeing their old light travelling toward you, ghosts of objects that no longer exist. From dead starlight to your own hand, you have never once touched the present moment, and you never will.
The catch
If perception always lagged, you would lose every fast-moving contest, yet people catch balls and dodge obstacles fine. The brain compensates by predicting forward. It does not just report where a moving object was when the light left it; it projects where the object should be now and shows you that. So your sense of "now" is part delayed reconstruction and part educated guess. It feels seamless and immediate precisely because your brain hides the delay from you. The present you experience is a controlled, convincing fiction built out of the past.