The lunar events of blue and black moons
C Sivaram, November 01, 2016
The phrase ‘once in a blue moon’ is often used in our daily conversations. As the phrase suggests, it indicates a truly rare event, that is, the occurrence of a second full moon in a calendar month.
On an average, this occurs once in nearly three years. But have you ever heard of a black moon? While blue moons are associated with full moons, black moons are associated with new moons. Indeed, the new moon that occurred on September 30, 2016, is an example of a black moon.
But to understand these two phenomena, you have to first know about the four principal phases of moon: the new moon, the first quarter moon, the full moon and the second quarter moon. When the moon moves in between these phases, we get waxing crescent, waxing gibbous, waning gibbous and waning crescent.
The occurrence of two new moons in a single calendar month is referred to as the black moon phenomenon. It is also a rare event as it happens about once in 32 months. A full moon is when the Earth-facing side of the moon is fully illuminated by the sunlight. But during a new moon, the Earth-facing side is fully in shadow.
In other words, new moon is the phase when its lighted half is facing entirely away from Earth (at new moon, the moon has the same celestial longitude as the Sun), which implies that the new moon is invisible or ‘black’. That is why the second new moon of the month is called the black moon while the occurrence of the second full moon is referred to as the blue moon.
At its new moon phase, the moon is always black. It happens at that time or the third month when the moon passes through the same part of the sky as the Sun and the moon’s dark or unilluminated side faces the Earth, so that there is really nothing to see! Of course, at times, the new moon passes directly between the Earth and the Sun and we can then see the moon’s black silhouette crossing in front of the Sun, leading to a spectacular solar eclipse, another rare event.
Rare phenomenon
In fact, this happened with the first new moon on September 1, 2016 and the annular solar eclipse called the ‘ring of fire’ eclipse occurred. This was visible over parts of Africa, amongst other places. So, the second new moon of September occurred on the last day of the month (September 30, 2016).
Officially, this event occurred on September, 30 at 8.11 pm (eastern time). However, for the Earth’s eastern hemisphere, the new moon occurred after midnight, corresponding to the calendar date of October 1, 2016. So, for this part of the world, this was the first new moon of October. For billions of people in the eastern hemisphere, the black moon or a second new moon of October occurred on October 30, 2016.
For the inhabitants of far eastern places like Japan or New Zealand, this didn’t happen till October 31, 2016. We had an unusual situation this year: the rare black moon phenomenon occurred on September 30, 2016, for people in the western hemisphere, but only on October 30 and 31, 2016, for the denizens of eastern hemisphere and far east.
Earlier, the term ‘blue moon’ was also used to describe the phenomenon caused by the high altitude dust in the Earth’s atmosphere, which scatters red light preferentially, making the moon appear blue. Blue moons are known to have been first seen in 1883 after the eruption of the Krakatoa volcano in Indonesia and then in 1950 due to forest fires in Canada. This phenomenon is also rare. In modern usage, it is the occurrence of two full moons in a calendar month.
It turns out that it is possible to have no full moon in a February containing 28 days. This is supposed to happen only four times in a century. It last occurred in 1999 and will next happen in 2018. Then in such a case, January and March will have two full moons. What is far more rare is a leap day (February 29) without a full moon. This freak event happed in 1608 and will not happen again till the year 2572 in the far-off future.
Looks like we have a lot to look forward to at the lunar front.
On an average, this occurs once in nearly three years. But have you ever heard of a black moon? While blue moons are associated with full moons, black moons are associated with new moons. Indeed, the new moon that occurred on September 30, 2016, is an example of a black moon.
But to understand these two phenomena, you have to first know about the four principal phases of moon: the new moon, the first quarter moon, the full moon and the second quarter moon. When the moon moves in between these phases, we get waxing crescent, waxing gibbous, waning gibbous and waning crescent.
The occurrence of two new moons in a single calendar month is referred to as the black moon phenomenon. It is also a rare event as it happens about once in 32 months. A full moon is when the Earth-facing side of the moon is fully illuminated by the sunlight. But during a new moon, the Earth-facing side is fully in shadow.
In other words, new moon is the phase when its lighted half is facing entirely away from Earth (at new moon, the moon has the same celestial longitude as the Sun), which implies that the new moon is invisible or ‘black’. That is why the second new moon of the month is called the black moon while the occurrence of the second full moon is referred to as the blue moon.
At its new moon phase, the moon is always black. It happens at that time or the third month when the moon passes through the same part of the sky as the Sun and the moon’s dark or unilluminated side faces the Earth, so that there is really nothing to see! Of course, at times, the new moon passes directly between the Earth and the Sun and we can then see the moon’s black silhouette crossing in front of the Sun, leading to a spectacular solar eclipse, another rare event.
Rare phenomenon
In fact, this happened with the first new moon on September 1, 2016 and the annular solar eclipse called the ‘ring of fire’ eclipse occurred. This was visible over parts of Africa, amongst other places. So, the second new moon of September occurred on the last day of the month (September 30, 2016).
Officially, this event occurred on September, 30 at 8.11 pm (eastern time). However, for the Earth’s eastern hemisphere, the new moon occurred after midnight, corresponding to the calendar date of October 1, 2016. So, for this part of the world, this was the first new moon of October. For billions of people in the eastern hemisphere, the black moon or a second new moon of October occurred on October 30, 2016.
For the inhabitants of far eastern places like Japan or New Zealand, this didn’t happen till October 31, 2016. We had an unusual situation this year: the rare black moon phenomenon occurred on September 30, 2016, for people in the western hemisphere, but only on October 30 and 31, 2016, for the denizens of eastern hemisphere and far east.
Earlier, the term ‘blue moon’ was also used to describe the phenomenon caused by the high altitude dust in the Earth’s atmosphere, which scatters red light preferentially, making the moon appear blue. Blue moons are known to have been first seen in 1883 after the eruption of the Krakatoa volcano in Indonesia and then in 1950 due to forest fires in Canada. This phenomenon is also rare. In modern usage, it is the occurrence of two full moons in a calendar month.
It turns out that it is possible to have no full moon in a February containing 28 days. This is supposed to happen only four times in a century. It last occurred in 1999 and will next happen in 2018. Then in such a case, January and March will have two full moons. What is far more rare is a leap day (February 29) without a full moon. This freak event happed in 1608 and will not happen again till the year 2572 in the far-off future.
Looks like we have a lot to look forward to at the lunar front.
No comments:
Post a Comment