In 1932, Jan Oort predicted dark matter to account for differences between mass calculated from astronomical objects’ gravitational effects and mass calculated from “luminous” matter contained in these objects (gas, stars, dust). In 1933, Fritz Zwicky observed that galaxies are moving too fast. In the Coma cluster, the gas is moving very fast, held at high temperatures. There must be a lot of gravity unseen to account for the pressure.
From 1965-1985, Vera Rubin discovered: 1. Rotation Curves – stars at the center and the edges travel at the same speeds, the closer the stars, the faster stars should travel, but evidence refuted this; 2. Gravitational Lensing – light is bent from the source as it travels to the observer.
Dark matter is believed to be a new class of subatomic particles. It cannot be seen or detected directly. Since it does not emit or absorb light and other electromagnetic waves, dark matter can only be predicted from its effects on visible matter. Astronomers believe dark matter account for 84% of matter and 23% of mass-energy in the Universe. Like “halos within halos,” dark matter surrounds galaxies, explaining such phenomena observed.
In the northern hemisphere in summer, the Sun rises in the northeast, stays high overhead at noon, and sets in the northwest. In the winter, the Sun rises in the southeast, stays low in the southern sky at noon, and sets in the southwest. Seasons are caused by the Earth’s tilt of rotational axis to the ecliptic and not by the Earth’s distance to the Sun. The Spring (Vernal) Equinox is the first day of spring (third week in March), when the Sun crosses the Celestial Equator the first time in the year. The Autumnal Equinox is the first day of Fall (third week of September), when the Sun crosses the Celestial Equator six months later. The Summer Solstice is the first day of summer (third week in June), the longest day of the year, and when the Sun is at its highest point in the ecliptic. The Winter Solstice is the first day of winter, the shortest day of the year, and when the Sun is at its lowest point in the ecliptic.