night sky

The moon is the major object that is viewed in the night sky. Do you know that they are many celestial objects that can be seen in the night sky? Let’s dig in and find out more.

LEARNING OBJECTIVES

By the end of this topic, you are expected to;

• Understand the meaning of night sky
• Know the different celestial objects that can be observed in a night sky

The term night sky is normally associated with astronomy from the Earth, and it refers to the appearance of celestial objects like the moon, planets, and stars at night time, which can be seen in a clear sky between sunset and sunrise. At this time the Sun is below the horizon.

The natural sources of light in a night sky include airglow, moonlight, and starlight, depending on timing and location. The skies that are above the polar circles are lighted by the aurorae. Occasionally, a large coronal mass ejection from high levels of solar wind or from the sun may extend the phenomenon toward the equator.

The night sky, as well as its studies, have a historical place in both modern and ancient cultures. For example, in the past, farmers used the status of the night sky as a calendar determining when to plant crops. Many cultures have also drawn constellations between the stars in the sky, associating them with legends and mythology about their deities.

The scientific study of the celestial objects that are visible at night takes place in the science of observational astronomy.

The visibility of the celestial objects at night is affected by light pollution. The presence of the moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient brightness. With the advent of artificial sources of light, however, light pollution has been a growing problem for viewing the night sky. Modification of light fixtures and optical filters can help alleviate this problem.

Brightness

The fact that the sky is not completely dark at night, even in the absence of city lights and moonlight, can be easily observed since if the sky was dark completely, it would be impossible for one to see the silhouette of an object against the sky.

The intensity of the sky is different over the day and the primary cause differs too. At daytime when the sun is above the horizon, direct scattering of sunlight is the overwhelmingly dominant source of light. In the twilight, which is the period of time between sunset and sunrise, the situation is more complicated and further differentiation is required. Depending on how far the sun is below the horizon, twilight is divided into three segments.

After sunset, the civil twilight sets in, and it ends when the sun drops more than 6⁰ below the horizon. This is followed by the nautical twilight, this is when the sun reaches heights of -6⁰ and -12⁰, after which comes the astronomical twilight that is defined as the period from -12⁰ to -18⁰. The general minimum brightness of the sky is attained when the sun drops more than 18⁰ below the horizon of the sky.

Several sources can be said to be the source of the intrinsic brightness of the sky, called airglow, scattering of starlight, indirect scattering of sunlight, and artificial light pollution.

Different celestial objects in the night sky

The main celestial objects that can be observed in a night sky include:

• Constellations. Constellations are prominent since their stars are brighter than nearby stars in the sky. Orion is among the major prominent and recognizable constellations.
• Planets. Venus is the most prominent planet in the night sky, often known as the morning star. It is brighter than the stars, therefore, making it very easy to be seen from earth. Depending on the location on its orbit, the planets Mercury, Mars, Jupiter, and Saturn can also be seen with a naked eye.
• The moon​​​​​​. The moon of the earth is a grey disc in the sky with cratering visible to the naked eye. The moon goes through a full cycle of lunar phases over 27.3 days. The different phases of the moon can be identified by simply looking at the moon. Unlike most planets and stars, the light reflected from the moon is bright enough to be seen during the day. The light reflected from the moon traveling through the atmosphere colors the moon orange and/or red.
• Comets. These come to the night sky only rarely. They are illuminated by the sun and their tails extend away from the sun. A great comet appears about once a decade. They are mainly visible shortly before sunrise or after sunset because those are the times they are close enough to the sun to show a tail.
• Clouds. Clouds obscure the view of other objects in the sky. This varies depending on the thickness of the cloud cover. A thin cirrus cloud in front of the moon might produce a rainbow-colored ring around the moon. Stars and planets are too dim or small to take on this effect.
• Milky way. The Milky Way can be seen as a very large streak or arc across the night sky if the visibility conditions are good enough.