An optical illusion is a perception that differs from physical reality, often caused by atmospheric effects or the brain's processing of visual information.

Size of the sun at dusk is an optical illusion: The sun appears larger near the horizon than when it's high in the sky, even though its actual angular size is unchanged. This is a psychological illusion caused by our brain misinterpreting distance cues.

Colour of the sun at dawn is an optical illusion: The sun appears red because its light travels through more of the atmosphere, which scatters away blue light. This phenomenon, known as Rayleigh scattering, creates a perception of color that differs from the sun's actual light.

Moon being visible at dawn is not an optical illusion: This is a real astronomical event. The moon is often in a position in its orbit where it is bright enough to be seen during daylight hours.

The twinkle of stars in the sky is an optical illusion: Stars don't actually blink. Their light is refracted by turbulent layers in Earth's atmosphere, causing their apparent brightness to change rapidly, which we perceive as twinkling.

Polestar being visible in the sky is not an optical illusion: This is a real phenomenon based on celestial mechanics. The Polestar's apparent fixed position is due to its close alignment with the Earth's axis of rotation