MOON-WATCH

The Moon was full on Jan. 28. The Moon reached the last quarter point on Feb. 4. The new Moon is Feb. 12.

PLANET-WATCH

Mercury has vanished into the solar glare and will not be visible again until later in the month.

Venus is a bright object, but it is not observable this month because it is in superior conjunction with the Sun, which is much brighter.

Venus will emerge from the twilight late this month, but will be difficult to see until mid-late March.

Mars will not be an easy object to observe low in the southwestern evening sky.

Jupiter will remain in our evening sky until late spring when it will set with the Sun.

Jupiter will be in the western sky throughout most of the evening. You can observe Jupiter’s apparent brightness diminish during the next few months as its distance from Earth increases.

Saturn is high in the eastern evening sky and will set in the pre-dawn western sky. Saturn resides in Taurus the Bull.

TWILIGHT ZONES

You don’t have to be a seasoned stargazer to realize that the sky doesn’t become dark right after sunset. There is a period of time between the time the sun sets and the onset of darkness: a period known as twilight.

We tend to associate twilight with an array of different colors: tangerine streaks blended with swaths of deep red and violet. As well as following sunset, twilight also precedes sunrise.

Twilight passes through three different phases each evening and in the pre-dawn. After sunset, we immediately have civil twilight which is followed by nautical twilight. Nautical twilight precedes astronomical twilight.

Before we offer the technical definition of each phase, we’ll give you a way to associate each phase with different observations: civil twilight is when the sky is ablaze with color; nautical twilight is when you start to see the brightest stars; astronomical twilight is when the fainter stars appear.

You’d be hard pressed to distinguish between the astronomical twilight and deep night.

To understand why twilight has three phases, we need to explain why our sky has a twilight period at all. Twilight is caused by the scattering of sunlight by the upper atmosphere. During the day, this scattering gives us a crisp blue sky. Yet, when the Sun is below the horizon, this scattering produces different colors because the light is passing through a greater amount of atmosphere.

The Sun’s light can be reflected by our atmosphere if our parent star is within 18 degrees of either horizon. The coloration of this scattering is most distinct when the Sun is within 6 degrees of the horizon. Thus, Civil twilight is defined as the time period when the Sun is within six degrees of the horizon (prior to sunrise and after sunset)

During this phase, car headlights, building lights, street lights come on: it is the phase that most directly affects our civil society, hence the term “civil twilight.”

When the Sun is between 6 to 12 degrees from the horizon, the sky darkens enough to allow us to see the brightest stars. The brightest stars are the ones most often used in celestial navigation.

When celestial navigators use night sky objects to ascertain their location, they need to perform their calculations during this phase.

During nautical twilight, the horizon is still visible, enabling the navigators to calculate the angular distance of planets and bright stars from the horizon line.

Astronomical twilight is the phase when the rest of the stars appear: Astronomical twilight is the period when the Sun is between 12 to 18 degrees away from the horizon. At this distance, light scattering still occurs, but to a far lesser degree.

After the Sun is 18 degrees away from the horizon, any light scattering is negligible.

The duration of twilight is a function of your latitude and the Sun’s declination (position relative to the celestial equator: it is highest on the summer solstice; lowest on the winter solstice).

Thus, the twilight duration changes from night to night.

However, as a rough rule of thumb, you can assume that each phase will last about half an hour. The Earth turns 15 degrees an hour: 6 degrees equals 24 minutes.

This rough approximation breaks apart as you venture near the Arctic or Antarctic Circles.

LEAVE A REPLY

Please enter your comment!
Please enter your name here