Place

Two Minute Astronomy, 6

Last week, we saw how the night sky changes with respect to time. We found that not only does the celestial sphere appear to rotate westward over the course of the day (as the Earth rotates), but also that the difference in solar and sidereal times causes the night sky to look different during different times of the year. Now, we will proceed to explore how the location you are stargazing from makes a difference.

In a previous chapter, we revisited latitudes and longitudes. Longitudes determine the time zone and the subsequent, but what role do latitudes play in astronomy? Recall that the celestial coordinate system is essentially the Earth’s own coordinate system ‘projected’ outward. Hence, the poles of the Earth and those of the celestial sphere are aligned, and the same is the case with the two equators. So, for a person at the Earth’s equator, the Celestial Equator lies straight above them (passes through the zenith, that is) and the Poles are at the horizon. For someone at, say, the North Pole, the North Celestial Pole is straight abovehead and the Celestial Equator straddles the horizon. At other latitudes, the height of the Celestial Pole (only one of them is visible from any place on Earth — except on the Equator) above the horizon will be equal to the latitude of the place you are viewing the sky from. The diagram below will aid in understanding this better.

The three scenarios described above. The red line is the horizon — the part of the sky below it is not visible to the viewer shown. (Image courtesy: openstax.org)

From the above diagram, it is clear how much of the sky we can view from a point on Earth. From the Earth’s Poles, only the corresponding hemisphere of the celestial sphere is visible, no matter the time. From the Equator, all parts of the celestial sphere are visible as it ‘rolls’ on its side. In other latitudes, one part of the sky is always visible and the diametrically opposite part is always obscured — say you are viewing the night sky from 10°N, then you’ll always see the stars that exist between 80°N and 90°N (the ‘top’ 10° of the celestial sphere) on the celestial sphere, but never see celestial objects that exist between 80°S and 90°S(the ‘bottom’ 10°). Stars in the intervening latitudes (80°N to 80°S) are visible sometimes. The stars that are visible all the time from a certain place are known as circumpolar stars.

So while the celestial sphere appears to rotate, different latitudes get a different vantage point, and hence, the sphere appears to rotate at different inclinations based on where you view the sky from.

The Celestial Poles are not exactly marked in any way in the night sky, but the North Pole is very close to the star we call Polaris in the constellation Ursa Minor. The South Celestial Pole has a very faint star called Sigma Octantis near it, but it is not readily visible to the naked eye.

A typical winter night in the mid-northern hemisphere would look like this. The brightest star in the night sky, Sirius, as well as Orion, one of the most conspicuous constellations, dominate the skyscape. (Image by Paul Schneider from Pixabay)

So up until now, we’ve laid the foundation to the basics of astronomy, exploring how the night sky changes and about the celestial sphere. We are now (finally!) ready to learn more about the celestial bodies themselves. Next week, we shall start with stars, their types and the life cycle they all go through. Until then, stay tuned!