Chapter 2.1
Earth in Space
How Earth's orbit & spin affects our view of the night sky
As Earth orbits the Sun, it moves around the host star by approximately one degree a day and at the same time is
completing one rotation every 23 hours and 56 minutes.
This is why we see the constellations shift westwards by one degree each night and rise in the east four minutes
earlier. Each season has its own sky, with its own prominent constellations - which is why Orion, for example,
dominates the winter sky but is absent in summer.
We can even see the effect of Earth's rotation within one evening or one hour. This apparent motion is often
captured by astrophotographers by taking non tracked long exposure images known as star trails.
To avoid star trails you can determine the maxiumum time for a single shot with the 500 rule if your equipment is not tracked.
You take the number 500 and divide by the focal length of your lens. For example, if you have a 20-mm wide angle lens, then 500 / 20 = 25. You can shoot for 25 seconds on a tripod before the stars start to streak.
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How to find planets? Locate the ecliptic
The first thing you need to do is find the ecliptic, the imaginary line that marks the path the Sun takes
across the sky.
Since all of the Solar System's major planets orbit the Sun in roughly the same plane, the ecliptic also
marks the path of the planets.
You will always find all of the planets near that line.
Is it bright, but on the wrong side of the sky to the ecliptic? Then it can't be a planet.
Keep an eye on the Moon, too. Its orbit around Earth tilts by about 5° compared to the ecliptic. That means
the ecliptic is always within 5° of the Moon.
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What effect does the axial tilt have on the Earth?
Earth's axis is not perpendicular. It has an axial tilt. This is the angle between the planet's rotational
axis and its orbital axis. Earth's axial tilt is about 23.44 degrees. Due to this, the sun shines on different
latitudes at different angles throughout the year. This causes the seasons.
More tilt means more severe seasons - warmer summers and longer days, colder winters and longer nights; less tilt means less severe seasons - cooler summers and shorter days, milder winters and shorter nights. No tilt means no seasons with equal days and nights.
Does Earth's tilted axis has influence on astrophotography? Yes, it might change what constellations are visible at a given time depending on the season.
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What does Equinoxes & Solstices mean for astronomy?
An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about
of equal length. The vernal equinox marks the start of spring, and the autumnal equinox marks the start of fall.
A solstice is one of the two times of the year resulting in the most amount of daylight time or the least amount
of daylight time in a single day. Solstices mark the start of summer and winter.
During the Summer solstice, the days are longer, and the nights are shorter. Summer is the perfect time for
terrestrial observation, But shorter nights limit the hours of darkness for astronomy.
On the other hand, during the Winter solstice, the days are shorter, and the nights are longer. Winter offers
astronomers ample time for celestial viewing, with the Sun setting early in the afternoon in some places.
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The Terrestrial Location System with Latitude & Longitude
This is the familiar system that locates positions on the Earth. The observer's location on Earth determines
the sky they will see. Latitude is a measurement of a location north or south of the Equator. In contrast,
longitude is a measurement of location east or west of the prime meridian at Greenwich.
Latitude and longitude together can describe the exact location of any place on Earth.
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