From Mercury, the sun appears larger and brighter compared to how it looks from Earth. Because Mercury is closer to the sun, the apparent size of the sun's disk in the sky is about 2.5 times larger than what we observe from Earth. This means the sun would take up a more significant portion of the sky on Mercury.

In terms of brightness, the sun appears more than twice as bright on Mercury compared to Earth. This increased brightness is due to Mercury's proximity to the sun, receiving a higher intensity of sunlight. The intensity of sunlight decreases with distance from the sun, so being closer to the sun results in a brighter appearance.

As Mercury orbits the sun, the changing phases of the sun can be observed from the planet's surface. Just like the phases of the moon, Mercury goes through phases such as new Mercury (when the side facing Earth is not lit by the sun) and full Mercury (when the side facing Earth is fully lit by the sun). These phases are caused by the relative positions of Mercury, Earth, and the sun as Mercury moves in its orbit.

The extreme temperatures on Mercury also play a role in how the sun appears. The intense heat and the presence of a thin atmosphere on Mercury can cause slight distortions and shimmering effects in the sun's image due to atmospheric turbulence and temperature gradients.

Overall, the sun viewed from Mercury would be significantly larger, brighter, and have distinct phases compared to what we experience on Earth.