Purdue University Calumet Electrical and Computer Engineering Technology ECET 521 Spring 2012 Chapter two Environmental Characteristics; By: Abdulrahman Alkhatlan One of the biggest stars on the Solar System is the sun, and it is in the center of our solar system. Sun's mass consists of hydrogen, while the rest is mostly helium, with a 1.39× 109 m, diameter with a temperature of 5760k; moreover, the sun sends a thermal radiation that travel with the speed of light, which it 300,000km/s, and that mean the sun solar energy can reaches earth with in 8 min and 20s (according to solar energy engineering - soteris kalogirou). So, with all this temperature and energy that equal to 3.8*10^20 MW earth receives only 1.7*10^14 kW, and that can get around 900EJ of energy. However, the advantages of getting to know the sun’s path through the sky is necessary to calculate the solar radiation falling on a surface, and to know how much of energy we can get from the sun and how can we work it to be a clean renewable energy, and how can we make the weather and the environment works for us. To calculate the solar energy we must use apparent solar time (AST), to direct the time of day, and AST use the apparent angular motion of the sun across the sky. Additionally, we can applied two correction to convert the AST to the local standard time (LST); which the equation of time and longitude correction. The equation of time ; the equation of time is not like the time we read in our watches, it depends on the time required by the earth to complete one revolution about its own axis with respect to the sun, to calculate the time according to the length of a day and its not uniform thru the year. We all know the day is 24 hours, but “the length of a day varies due to the eccentricity of the earth’s orbit and the tilt of the earth’s axis from the normal plane of its orbit” (According to solar energy engineering - soteris kalogirou).And because this ellipticity of the orbit the earth orbiting speed can change through the year, as in October to March earth orbiting become faster then average speed and slower from April to September. “ This equations taken from the book solar energy engineering - soteris kalogirou” “The values of the equation of time as a function of the day of the year (N) can be obtained approximately from the following equations: Solar angles; The plane that includes the earth 's equator is called the equatorial plane. If a line is drawn between the center of the earth and the sun, the angle between this line and the earth's equatorial plane is called the declination angle. At the time of year when the northern part of the earth's rotational axis is inclined toward the sun, the earth 's equatorial plane is inclined 23.45 degrees to the earth-sun line. At this time (about June 21), we observe that the noontime sun is at its highest point in the sky and the declination angle = +23.45 degrees. We call this condition the summer solstice, and it marks the beginning of summer in the Northern Hemisphere. As the earth continues its yearly orbit about the sun, a point is reached about 3 months later where a line from the earth to the sun lies on the equatorial plane. At this point an observer on the equator would observe that the sun was directly overhead at noontime. This condition is called an equinox since anywhere on the earth, the time during which the sun is visible (daytime) is exactly 12 hours and the time when it is not visible (nighttime) is 12 hours. There are two such conditions during a year; the autumnal equinox on about September 23, marking the start of the fall; and the vernal equinox on about March 22, marking the beginning of spring. At the equinoxes, the declination angle is zero. The winter solstice occurs on about December 22 and marks the point where the equatorial plane is tilted relative to the earth-sun line such that the northern hemisphere is tilted away from the sun. We say that the noontime sun is at its "lowest point" in the sky, meaning that the declination angle is at its most negative value = -23.45 degrees. By convention, winter declination angles are negative. Accurate knowledge of the declination angle is important in navigation and astronomy. Very accurate values are published annually in tabulated form in an ephemeris Where the argument of the cosine here is in degrees and N is the day number defined fort his Equation When we observe the sun from an arbitrary position on the earth, we are interested in defining the sun position relative to a coordinate system based at the point of observation, not at the center of the earth. The conventional earth-surface based coordinates are a vertical line (straight up) and a horizontal plane containing a north-south line and an east-west line. The position of the sun relative to these coordinates can be described by two angles; the solar altitude angle and the solar zenith angle defined.