The Pole Star's Visibility at the Earth's Equator

For an observer situated on the Earth's equator, the Pole Star, commonly known as Polaris, appears directly on the northern horizon. This position results from Polaris's location nearly at the celestial North Pole, causing it to sit at an altitude of 0 degrees above the horizon for an observer on the equator.

As one moves away from the equator towards the North Pole, Polaris rises higher in the sky, reaching an altitude of 90 degrees directly overhead. Conversely, traveling towards the South Pole would result in Polaris disappearing from view. However, it is important to note that Polaris is actually 0.66 degrees off from the Earth's rotational axis, which slightly alters its apparent position.

Visual Path of the Pole Star at the Equator

If you were standing at the Earth's equator at sea level, surrounded by calm ocean waters, and ignoring atmospheric refraction, you would see Polaris follow a path above the horizon half the time. Specifically, Polaris would ascend east of due north by 0.66 degrees, reach its maximum altitude of 0.66 degrees directly north, and then set about 0.66 degrees west of north. This tiny arc ensures that an observer would rarely see Polaris at its nadir (lowest point).

Impact of Atmospheric Refraction

Including atmospheric refraction, a phenomenon that bends light due to temperature gradients in the atmosphere, would extend the time Polaris remains visible above the apparent horizon to approximately 22 hours a day. However, this visibility can be disrupted by various atmospheric conditions and natural elements such as clouds, sunlight, and ocean waves.

At the horizon, an observer must consider the substantial number of air masses (around 40) that the light must pass through, even on a clear night. Each airmass transmits less than 92% of the light, reducing the total light transmission significantly. Under such conditions, Polaris would be barely visible to the naked eye, and for optimal visibility, one might need a telescope or binoculars on a dark, clear, moonless night. At other times, spotting Polaris would require a larger telescope and considerable luck.

Another noteworthy aspect is the differential absorption of light wavelengths by the atmosphere. Blue light is attenuated much more rapidly than red light, which is evident in the red appearance of the Sun near the horizon. This phenomenon applies equally to the visibility of celestial objects like the Pole Star, with blue light being obscured more easily than red light.

For an observer at the horizon, the apparent shift caused by atmospheric refraction means Polaris would actually be right on the horizon or up to a degree above or below it, depending on the time of day and the time of year. Nonetheless, atmospheric refraction ensures that Polaris remains visible above the horizon, enhancing its visibility in the night sky.