PHY115: Professional Skills in Physics and Astronomy

Positional Astronomy: Session 2

Predicting the Sun's RA and Dec

We can use the Sun's motion around the ecliptic to calculate the right ascension (RA), and declination (dec) of the Sun on any given day. Around March 20th, the Sun is at the first point of Aries and has zero dec, and zero RA. Since it is moving from south to north at this point, the dec of the Sun will increase with time. The dec of the Sun reaches a maximum of +ε at the summer solstice, and a minimum of -ε at the winter solstice. What happens to the Sun's right ascension? Imagine the Sun at noon on the vernal equinox. The Sun is on the observer's meridian. One sidereal day later, all the stars are in the same position. However, since the sidereal day is shorter than the solar day, the Sun has not yet reached the meridian; it is slightly to the east. Since RA increases to the east, it follows that the Sun's RA has increased. Over the course of a year the Sun must move through 24h of RA to return to the same place, so the Sun's RA increases by roughly 2 hours a month. The applet below summarises the motion of the Sun in RA and dec, over the course of a year.

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Now we know how the Sun's RA and Dec change over the course of a year, we can see how the Sun's position in the observer's sky changes. The animation below shows the position of the Sun at local noon, over the course of a year. Using the information above, you should be able to understand both the Sun's position at noon, and it's daily path in the sky.

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