Your Nighttime Sky

Say you want to go out into your backyard and look at the night sky. How do you know what you will be able to see when you look up? It all depends on two things: your location on Earth and the time of year.


First, let's define the position of celestial objects in the sky. Anything that is not a solar system body has a position determined by its location on the celestial sphere. They are the right ascension and the declination.

• Right Ascension (RA) is similar to longitudes on Earth, but are measure in hours from 0 to 24. 0h RA is defined as the line that contains the vernal equinox and RA increases as you go east along the celestial equator
• Declination (Dec) is similar to latitudes on Earth and are measured in the same way. 0° Dec is at the celestial equator and increases as you go north or south along a RA to 90°. In astronomy, +90° is the north celestial pole and -90° is the south celestial pole.

Your zenith is the point directly above your head on the celestial sphere. The zenith will always depend on your location on Earth. So a zenith for someone in Pittsburgh will be different from someone in Kuala Lumpur. The declination of your zenith will always be the same as your latitude. If you draw a semicircle north-south that includes your zenith, you get an imaginary arcs that can be used to define a local coordinate plane.

• The north-south line is called your local meridian. Any object that lies on your meridian will have the same RA at that time. However, at different times, due to the rotation of the Earth, the meridian's RA will change. What we use is something called the hour angle to describe an objects position in the sky relative to meridian's RA. The hour angle is defined as how far east or west of the meridian an object is in the sky. It's the local sidereal time minus the right ascension of the object you are observing. If the hour angle is negative, the object is to the east of the meridian. If the hour angle is positive, it is to the west.

Your latitude is also important. By definition, if any object is more than 90° from your zenith, it should be visible, assuming no obstructions lie on the horizon. For example, Pittsburgh is at 40°N. By definition any object that is north of -50° declination should appear in the sky sometime during the year. (In reality, because the horizon is obscured in most locations, the declination is closer to -40° for objects to appear.) There is also a unique feature of any objects that are within 40° of the north celestial pole. These objects are called circumpolar because they never rise nor set in the sky (you can't see them during the day, but if you took a time lapsed image of stars that are circumpolar, they would make an arc around the north celestial pole).

Another way you can describe an object's position in your local sky, is by using the altitude and azimuth of the object. The altitude is just how far above the horizon an object is and is measured in degrees (the altitude of the zenith is 90°). The azimuth is how far east or west of due north or due south an object is (also measured in degrees). Most astronomers use the angle from due north so an object can have an azimuth of greater than 90°. There is a caveat to using altitude and azimuth, however. These are only good for your location. If you are trying to coordinate with another observer elsewhere, you should always use an object's RA and Declination.

0.07%

0.07%


That's a small number.


7 parts in 10,000.


But this small fraction is a significant number in terms of the evolution of our universe. This is the mass fraction difference between one helium nucleus and four hydrogen nuclei. During the proton-proton chain, four protons (essentially, the nucleus of a hydrogen atom is just a proton) are fused into one helium nucleus (two protons and two neutrons). The helium nucleus is smaller than four hydrogen nuclei by only 0.07%. This extra 0.07% mass is converted to energy via E=mc².


This fraction may seem insignificant, but in reality, it is very important. If the mass fraction was only 0.06%, then stars would take too long to evolve and may even be unstable. The energy released by the proton-proton chain would not be enough to hold back gravitational forces from the outer layers of the star, and the star would collapse.*


*The balance between the energy from fusion in the core of a star and the gravity of the star pushing in is called hydrostatic equilibrium. This is why stars are stable. It is also why stars tend to expand and contract. As the star evolves, the fusion in the core increases as it begins to use up its fuel and the star expands as it evolves on the main sequence. When fusion stages end (at the end of the Main Sequence, for example), gravity will overcome the fusion energy and push in. See more about the evolution of a star here.


If the mass fraction was just a tad higher, at 0.08%, the fusion of hydrogen into helium would occur too fast and stars would use up their fuel too quickly (and possible even dissipate the outer layers of the star as the energy would overcome gravity and push out the gases in the outer layers). If the stars were stable, the stars would use up their mass too quickly for planets to form and in turn, life would probably not evolve.

Sidereal Day Vs. Synodic Day

I’ve talked here before about sidereal days and synodic days. What exactly is the difference between the two?

It all comes down to one thing: the object that you are using as a reference. For example, the sidereal day of the Earth is equal to 23 hours, 56 minutes, and 4 seconds. The sidereal day uses a distant star (which is basically any star that is not the Sun) to measure against. You would determine how long a star would take to reach the same location in the sky on consecutive days. Another way to look at it, is the length of time it takes the Earth to rotate 360°.

However, the synodic day is 24 hours. The best way to think of the synodic day is to just determine the length of time between two consecutive noons. A synodic day means the Earth rotates more than 360° to get the Sun in the same location in the sky. The reason this happens is because as the Earth rotates on its axis, it is also moving around the Sun. Granted, it is only moving a little less than a degree in its orbit (since the Earth year is about 365.25 days), but that is long enough that the Earth has to rotate just a tad more to get the Sun back to noon.