Figure 5 shows the effect on Polaris at an altitude of 52 o. This effect, only really visible in a telescope, increases as an object’s altitude decreases. This can cause a star’s image to be spread out into a very small spectrum with red at the top and blue at the bottom. Most methods that use star alignment will do this automatically but if you are using a mechanical method such as an inclinometer you could make an adjustment for refraction particularly at low latitudes.Ītmospheric Dispersion At the beginning of this tutorial we noted that the amount light is refracted varies slightly with its wavelength (colour). With an equatorial mount that needs to be aligned on the celestial pole the best thing to do is to align on the refracted pole not the geometric one.
For purely visual observing this is something that is easy to cope with but for photography it does present a challenge. The continually changing amount of refraction means that the telescope will not keep a star perfectly centred in the field of view. Consequently its path across the sky is more complex.ĭoes this matter? The answer is yes if you have a telescope drive that is attempting to track the stars. Taking refraction into account we find that any star, except at the zenith, is higher than it would be and the difference varies with its altitude. This is a reasonable approximation to reality and is perfectly correct in the absence of the atmosphere.
This is usually shown as a smooth curve with the star rising in the eastern part of the sky, reaching its highest point on the meridian and then sinking towards the western horizon. The same effect can be seen with the Moon.Ĭonsider the case of a star moving across the sky. Also local conditions can cause the amount of refraction to vary significantly at such low altitudes. This is very much the ideal situation in reality discrete layers in the atmosphere can have different effects often causing a jagged edge to the solar limb. With the refraction at the Sun's lower limb being greater than that at the upper, the lower limb is uplifted by about 16% more than the upper and the normally round solar disk is flattened into an oval shape.
Because the altitudes differ by 0.5 o which is about the Sun’s apparent diameter, the lower limb of the Sun is lifted by roughly 34’ and the upper by ‘only’ 29’. These show the position when Sun seems on to be on the horizon, but is actually below it. In particular, the day becomes longer with the Sun appearing to rise earlier and set later than it would in the absence of an atmosphere.Ĭonsider the last two entries in the refraction table above. The effect of all this is to lengthen the time an object is above the horizon. Obviously this uplift applies to any astronomical object such as stars, planets and the Moon, not just the Sun. If we could miraculously make the Earth’s atmosphere vanish at the apparent moment of sunset or sunrise then the Sun would actually disappear from view as it is really already below the horizon. When the Sun appears to be sitting on the horizon, just rising or setting, it is really below the geometric horizon and is only lifted into visibility by atmospheric refraction. As it approaches the horizon the amount of refraction is around 0.5 o, which is roughly the Sun's apparent diameter. On the horizon Let's consider the setting Sun.