And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
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Basil the Great
Since the birth of the sun, the light that it diffuses in the air, when shining on our hemisphere, is day; and the shadow produced by its disappearance is night. But at that time it was not after the movement of the sun, but following this primitive light spread abroad in the air or withdrawn in a measure determined by God, that day came and was followed by night.
And the evening and the morning were the first day. Genesis 1:5 Evening is then the boundary common to day and night; and in the same way morning constitutes the approach of night to day. It was to give day the privileges of seniority that Scripture put the end of the first day before that of the first night, because night follows day: for, before the creation of light, the world was not in night, but in darkness. It is the opposite of day which was called night, and it did not receive its name until after day. Thus were created the evening and the morning. Scripture means the space of a day and a night, and afterwards no more says day and night, but calls them both under the name of the more important: a custom which you will find throughout Scripture. Everywhere the measure of time is counted by days, without mention of nights. The days of our years, says the Psalmist. Few and evil have the days of the years of my life been, Genesis 47:9 said Jacob, and elsewhere all the days of my life. Thus under the form of history the law is laid down for what is to follow. And the evening and the morning were one day. Why does Scripture say one day the first day? Before speaking to us of the second, the third, and the fourth days, would it not have been more natural to call that one the first which began the series? If it therefore says one day, it is from a wish to determine the measure of day and night, and to combine the time that they contain. Now twenty-four hours fill up the space of one day— we mean of a day and of a night; and if, at the time of the solstices, they have not both an equal length, the time marked by Scripture does not the less circumscribe their duration. It is as though it said: twenty-four hours measure the space of a day, or that, in reality a day is the time that the heavens starting from one point take to return there. Thus, every time that, in the revolution of the sun, evening and morning occupy the world, their periodical succession never exceeds the space of one day. But must we believe in a mysterious reason for this? God who made the nature of time measured it out and determined it by intervals of days; and, wishing to give it a week as a measure, he ordered the week to revolve from period to period upon itself, to count the movement of time, forming the week of one day revolving seven times upon itself: a proper circle begins and ends with itself. Such is also the character of eternity, to revolve upon itself and to end nowhere. If then the beginning of time is called one day rather than the first day, it is because Scripture wishes to establish its relationship with eternity. It was, in reality, fit and natural to call one the day whose character is to be one wholly separated and isolated from all the others. If Scripture speaks to us of many ages, saying everywhere, age of age, and ages of ages, we do not see it enumerate them as first, second, and third. It follows that we are hereby shown not so much limits, ends and succession of ages, as distinctions between various states and modes of action. The day of the Lord, Scripture says, is great and very terrible, Joel 2:11 and elsewhere Woe unto you that desire the day of the Lord: to what end is it for you? The day of the Lord is darkness and not light. Amos 5:18 A day of darkness for those who are worthy of darkness. No; this day without evening, without succession and without end is not unknown to Scripture, and it is the day that the Psalmist calls the eighth day, because it is outside this time of weeks. Thus whether you call it day, or whether you call it eternity, you express the same idea. Give this state the name of day; there are not several, but only one. If you call it eternity still it is unique and not manifold. Thus it is in order that you may carry your thoughts forward towards a future life, that Scripture marks by the word one the day which is the type of eternity, the first fruits of days, the contemporary of light, the holy Lord's day honoured by the Resurrection of our Lord. And the evening and the morning were one day.
But, while I am conversing with you about the first evening of the world, evening takes me by surprise, and puts an end to my discourse. May the Father of the true light, Who has adorned day with celestial light, Who has made the fire to shine which illuminates us during the night, Who reserves for us in the peace of a future age a spiritual and everlasting light, enlighten your hearts in the knowledge of truth, keep you from stumbling, and grant that you may walk honestly as in the day. Romans 13:13 Thus shall you shine as the sun in the midst of the glory of the saints, and I shall glory in you in the day of Christ, to Whom belong all glory and power for ever and ever. Amen.